The existing docs states that, get looks for a branch or tag
that matches the locally installed version of Go.
First, this is only working for "go1", so it could be confusing.
Second, "If no such version exists it retrieves the most recent
version of the package". It's more the default branch, by git defaults,
rather than most recent version.

This should address the potential unclear parts.

Fixes #20320

Change-Id: Id7d727d88dc350c9902974b64fa28c3766f7e245
Reviewed-on: https://go-review.googlesource.com/45890Reviewed-by: Ian Lance Taylor <iant@golang.org>