[release-branch.go1.13] doc/go1.13: add information about using private modules to the introduction

Updates #33796
Fixes #33846

Change-Id: I9f6837be96410e96d004523c48bef95ee1427484
Reviewed-on: https://go-review.googlesource.com/c/go/+/191746


Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 989409f5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/191749


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>