Expand modules documentation to clarify why @none is useful. The
wording is the one suggested by rsc on the issue.

Fixes #26684

Change-Id: I76dc4ff87e50f1dd8536fd9ac1fd938adb29bee3
Reviewed-on: https://go-review.googlesource.com/c/161037Reviewed-by: Bryan C. Mills <bcmills@google.com>