cmd/go: explain x... vs. x/... in help importpath

Fixes #3110.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5696083

src/cmd/go/help.go

@@ -34,8 +34,9 @@ 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
 directories found in the GOPATH trees with names matching the
-patterns.  For example, encoding/... expands to all packages
-in the encoding tree.
+patterns.  For example, encoding/... expands to all package
+in subdirectories of the encoding tree, while net... expands to
+net and all its subdirectories.
 
 An import path can also name a package to be downloaded from
 a remote repository.  Run 'go help remote' for details.