all: document vendoring in the standard library

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>