os: document Readlink behavior for relative links

os: document Readlink behavior for relative links

Also provide a runnable example to illustrate that behavior.

This should help users to avoid the common mistake of expecting
os.Readlink to return an absolute path.

Fixes #57766.

Change-Id: I8f60aa111ebda0cae985758615019aaf26d5cb41
Reviewed-on: https://go-review.googlesource.com/c/go/+/546995
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Rob Pike <r@golang.org>