cmd/go: document that cmd/foo is only in Go repo, never GOPATH

It's always been like this, so document it.

Fixes #14351.

Change-Id: Ic6a7c44881bac0209fa6863a487fabec5ec0214e
Reviewed-on: https://go-review.googlesource.com/31663
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/help.go

@@ -62,6 +62,9 @@ Go library.
 - "cmd" expands to the Go repository's commands and their
 internal libraries.
 
+Import paths beginning with "cmd/" only match source code in
+the Go repository.
+
 An import path is a pattern if it includes one or more "..." wildcards,
 each of which can match any string, including the empty string and
 strings containing slashes.  Such a pattern expands to all package