cmd/go: document that package documentation is ignored

Fixes #11801.

Change-Id: I2caeac7fdddc7f29015d6db8d4b3e296c8b9c423
Reviewed-on: https://go-review.googlesource.com/17954Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/help.go

@@ -79,6 +79,14 @@ internally at Google all begin with 'google', and paths
 denoting remote repositories begin with the path to the code,
 such as 'github.com/user/repo'.
 
+Packages in a program need not have unique package names,
+but there are two reserved package names with special meaning.
+The name main indicates a command, not a library.
+Commands are built into binaries and cannot be imported.
+The name documentation indicates documentation for
+a non-Go program in the directory. Files in package documentation
+are ignored by the go command.
+
 As a special case, if the package list is a list of .go files from a
 single directory, the command is applied to a single synthesized
 package made up of exactly those files, ignoring any build constraints