doc: add section on resolving packages to module reference doc

Updates #33637

Change-Id: I3b4303479b44894442a04226cbf1f1e27dbb2fb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/208779


Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>