doc: add section for major version suffixes to module reference

Updates #33637

Change-Id: Ieb8fce1b9c44f630cddc5ff6d19daa17185867e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/206618


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>