Added documentation that explains special cases for vendored packages
in the standard library and provides instructions for updating vendor
directories.
Fixes#31806
Change-Id: Ib697ed18eae28023ab0bfb9f4d250992c393571d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174999
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Jay Conrod