all: freeze net/rpc and reword the 'frozen' message in other frozen packages

Make the messages grammatically korrect and consistent.

Fixes #16844

Change-Id: I7c137b4dc25c0c875ed07b0c64c67ae984c39cbc
Reviewed-on: https://go-review.googlesource.com/32112
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/log/syslog/doc.go

@@ -10,7 +10,7 @@
 // the syslog client will attempt to reconnect to the server
 // and write again.
 //
-// The syslog package is frozen and not accepting new features.
+// The syslog package is frozen and is not accepting new features.
 // Some external packages provide more functionality. See:
 //
 //   https://godoc.org/?q=syslog

src/net/rpc/server.go

@@ -121,6 +121,8 @@
 
 	A server implementation will often provide a simple, type-safe wrapper for the
 	client.
+
+	The net/rpc package is frozen and is not accepting new features.
 */
 package rpc
 

src/net/smtp/smtp.go

@@ -9,7 +9,7 @@
 //	STARTTLS  RFC 3207
 // Additional extensions may be handled by clients.
 //
-// The smtp package is frozen and not accepting new features.
+// The smtp package is frozen and is not accepting new features.
 // Some external packages provide more functionality. See:
 //
 //   https://godoc.org/?q=smtp

src/testing/quick/quick.go

@@ -4,7 +4,7 @@
 
 // Package quick implements utility functions to help with black box testing.
 //
-// The package is frozen and not accepting new features.
+// The testing/quick package is frozen and is not accepting new features.
 package quick
 
 import (

src/text/tabwriter/tabwriter.go

@@ -8,7 +8,7 @@
 // The package is using the Elastic Tabstops algorithm described at
 // http://nickgravgaard.com/elastictabstops/index.html.
 //
-// The package is frozen and not accepting new features.
+// The text/tabwriter package is frozen and is not accepting new features.
 package tabwriter
 
 import (