cmd/go: multiple small 'go help' fixes

* Link to privacy policies for proxy.golang.org and sum.golang.org in
  'go help modules'. It's important that both policies are linked from
  the go command's documentation.
* Fix wording and typo in 'go help vcs' following comments in CL 290992,
  which adds reference documentation for GOVCS.
* Fix whitespace on GOVCS in 'go help environment'.

For #41730

Change-Id: I86abceacd4962b748361244026f219157c9285e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/291230
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -2410,6 +2410,17 @@
 //
 // For a detailed reference on modules, see https://golang.org/ref/mod.
 //
+// By default, the go command may download modules from https://proxy.golang.org.
+// It may authenticate modules using the checksum database at
+// https://sum.golang.org. Both services are operated by the Go team at Google.
+// The privacy policies for these services are available at
+// https://proxy.golang.org/privacy and https://sum.golang.org/privacy,
+// respectively.
+//
+// The go command's download behavior may be configured using GOPROXY, GOSUMDB,
+// GOPRIVATE, and other environment variables. See 'go help environment'
+// and https://golang.org/ref/mod#private-module-privacy for more information.
+//
 //
 // Module authentication using go.sum
 //
@@ -2868,20 +2879,23 @@
 // legal reasons). Therefore, clients can still access public code served from
 // Bazaar, Fossil, or Subversion repositories by default, because those downloads
 // use the Go module mirror, which takes on the security risk of running the
-// version control commands, using a custom sandbox.
+// version control commands using a custom sandbox.
 //
 // The GOVCS variable can be used to change the allowed version control systems
 // for specific packages (identified by a module or import path).
-// The GOVCS variable applies both when using modules and when using GOPATH.
+// The GOVCS variable applies when building package in both module-aware mode
-// When using modules, the patterns match against the module path.
+// and GOPATH mode. When using modules, the patterns match against the module path.
-// When using GOPATH, the patterns match against the import path
+// When using GOPATH, the patterns match against the import path corresponding to
-// corresponding to the root of the version control repository.
+// the root of the version control repository.
 //
 // The general form of the GOVCS setting is a comma-separated list of
 // pattern:vcslist rules. The pattern is a glob pattern that must match
 // one or more leading elements of the module or import path. The vcslist
 // is a pipe-separated list of allowed version control commands, or "all"
-// to allow use of any known command, or "off" to allow nothing.
+// to allow use of any known command, or "off" to disallow all commands.
+// Note that if a module matches a pattern with vcslist "off", it may still be
+// downloaded if the origin server uses the "mod" scheme, which instructs the
+// go command to download the module using the GOPROXY protocol.
 // The earliest matching pattern in the list applies, even if later patterns
 // might also match.
 //
@@ -2889,7 +2903,7 @@
 //
 // 	GOVCS=github.com:git,evil.com:off,*:git|hg
 //
-// With this setting, code with an module or import path beginning with
+// With this setting, code with a module or import path beginning with
 // github.com/ can only use git; paths on evil.com cannot use any version
 // control command, and all other paths (* matches everything) can use
 // only git or hg.

src/cmd/go/internal/modget/get.go

@@ -176,20 +176,23 @@ packages or when the mirror refuses to serve a public package (typically for
 legal reasons). Therefore, clients can still access public code served from
 Bazaar, Fossil, or Subversion repositories by default, because those downloads
 use the Go module mirror, which takes on the security risk of running the
-version control commands, using a custom sandbox.
+version control commands using a custom sandbox.
 
 The GOVCS variable can be used to change the allowed version control systems
 for specific packages (identified by a module or import path).
-The GOVCS variable applies both when using modules and when using GOPATH.
+The GOVCS variable applies when building package in both module-aware mode
-When using modules, the patterns match against the module path.
+and GOPATH mode. When using modules, the patterns match against the module path.
-When using GOPATH, the patterns match against the import path
+When using GOPATH, the patterns match against the import path corresponding to
-corresponding to the root of the version control repository.
+the root of the version control repository.
 
 The general form of the GOVCS setting is a comma-separated list of
 pattern:vcslist rules. The pattern is a glob pattern that must match
 one or more leading elements of the module or import path. The vcslist
 is a pipe-separated list of allowed version control commands, or "all"
-to allow use of any known command, or "off" to allow nothing.
+to allow use of any known command, or "off" to disallow all commands.
+Note that if a module matches a pattern with vcslist "off", it may still be
+downloaded if the origin server uses the "mod" scheme, which instructs the
+go command to download the module using the GOPROXY protocol.
 The earliest matching pattern in the list applies, even if later patterns
 might also match.
 
@@ -197,7 +200,7 @@ For example, consider:
 
 	GOVCS=github.com:git,evil.com:off,*:git|hg
 
-With this setting, code with an module or import path beginning with
+With this setting, code with a module or import path beginning with
 github.com/ can only use git; paths on evil.com cannot use any version
 control command, and all other paths (* matches everything) can use
 only git or hg.

src/cmd/go/internal/modload/help.go

@@ -6,8 +6,6 @@ package modload
 
 import "cmd/go/internal/base"
 
-// TODO(rsc): The "module code layout" section needs to be written.
-
 var HelpModules = &base.Command{
 	UsageLine: "modules",
 	Short:     "modules, module versions, and more",
@@ -22,6 +20,17 @@ For a series of tutorials on modules, see
 https://golang.org/doc/tutorial/create-module.
 
 For a detailed reference on modules, see https://golang.org/ref/mod.
+
+By default, the go command may download modules from https://proxy.golang.org.
+It may authenticate modules using the checksum database at
+https://sum.golang.org. Both services are operated by the Go team at Google.
+The privacy policies for these services are available at
+https://proxy.golang.org/privacy and https://sum.golang.org/privacy,
+respectively.
+
+The go command's download behavior may be configured using GOPROXY, GOSUMDB,
+GOPRIVATE, and other environment variables. See 'go help environment'
+and https://golang.org/ref/mod#private-module-privacy for more information.
 	`,
 }