cmd/compile: improve compiler directive docs

This section has gotten long enough that it deserves to be multiple
sections. This also allows us to better structure information shared
by subsets of directives. In particular, this enables a self-contained
section on the wasm directives.

Updates #66984.

Change-Id: I062081d46c6b0aef7887fdaf9efae80f32ad4b21
Reviewed-on: https://go-review.googlesource.com/c/go/+/638881
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/doc.go

@@ -15,7 +15,7 @@ the package and about types used by symbols imported by the package from
 other packages. It is therefore not necessary when compiling client C of
 package P to read the files of P's dependencies, only the compiled output of P.
 
-Command Line
+# Command Line
 
 Usage:
 
@@ -150,14 +150,21 @@ Flags to debug the compiler itself:
 	-w
 		Debug type checking.
 
-Compiler Directives
+# Compiler Directives
 
 The compiler accepts directives in the form of comments.
-To distinguish them from non-directive comments, directives
+Each directive must be placed its own line, with only leading spaces and tabs
-require no space between the comment opening and the name of the directive. However, since
+allowed before the comment, and there must be no space between the comment
-they are comments, tools unaware of the directive convention or of a particular
+opening and the name of the directive, to distinguish it from a regular comment.
+Tools unaware of the directive convention or of a particular
 directive can skip over a directive like any other comment.
+
+Other than the line directive, which is a historical special case;
+all other compiler directives are of the form
+//go:name, indicating that they are defined by the Go toolchain.
 */
+// # Line Directives
+//
 // Line directives come in several forms:
 //
 // 	//line :line
@@ -197,12 +204,9 @@ directive can skip over a directive like any other comment.
 // Line directives typically appear in machine-generated code, so that compilers and debuggers
 // will report positions in the original input to the generator.
 /*
-The line directive is a historical special case; all other directives are of the form
+# Function Directives
-//go:name, indicating that they are defined by the Go toolchain.
+
-Each directive must be placed its own line, with only leading spaces and tabs
+A function directive applies to the Go function that immediately follows it.
-allowed before the comment.
-Each directive applies to the Go code that immediately follows it,
-which typically must be a declaration.
 
 	//go:noescape
 
@@ -245,6 +249,8 @@ It specifies that the function must omit its usual stack overflow check.
 This is most commonly used by low-level runtime code invoked
 at times when it is unsafe for the calling goroutine to be preempted.
 
+# Linkname Directive
+
 	//go:linkname localname [importpath.name]
 
 The //go:linkname directive conventionally precedes the var or func
@@ -295,17 +301,34 @@ The declaration of lower.f may also have a linkname directive with a
 single argument, f. This is optional, but helps alert the reader that
 the function is accessed from outside the package.
 
+# WebAssembly Directives
+
 	//go:wasmimport importmodule importname
 
 The //go:wasmimport directive is wasm-only and must be followed by a
-function declaration.
+function declaration with no body.
 It specifies that the function is provided by a wasm module identified
-by ``importmodule`` and ``importname``.
+by ``importmodule'' and ``importname''. For example,
 
 	//go:wasmimport a_module f
 	func g()
 
-The types of parameters and return values to the Go function are translated to
+causes g to refer to the WebAssembly function f from module a_module.
+
+	//go:wasmexport exportname
+
+The //go:wasmexport directive is wasm-only and must be followed by a
+function definition.
+It specifies that the function is exported to the wasm host as ``exportname''.
+For example,
+
+	//go:wasmexport h
+	func hWasm() { ... }
+
+make Go function hWasm available outside this WebAssembly module as h.
+
+For both go:wasmimport and go:wasmexport,
+the types of parameters and return values to the Go function are translated to
 Wasm according to the following table:
 
     Go types        Wasm types
@@ -318,24 +341,12 @@ Wasm according to the following table:
     pointer         i32 (more restrictions below)
     string          (i32, i32) (only permitted as a parameters, not a result)
 
+Any other parameter types are disallowed by the compiler.
+
 For a pointer type, its element type must be a bool, int8, uint8, int16, uint16,
 int32, uint32, int64, uint64, float32, float64, an array whose element type is
 a permitted pointer element type, or a struct, which, if non-empty, embeds
-structs.HostLayout, and contains only fields whose types are permitted pointer
+[structs.HostLayout], and contains only fields whose types are permitted pointer
 element types.
-
-Any other parameter types are disallowed by the compiler.
-
-	//go:wasmexport exportname
-
-The //go:wasmexport directive is wasm-only and must be followed by a
-function definition.
-It specifies that the function is exported to the wasm host as ``exportname``.
-
-	//go:wasmexport f
-	func g()
-
-The types of parameters and return values to the Go function are permitted and
-translated to Wasm in the same way as //go:wasmimport functions.
 */
 package main