doc/go1.13: add information about using private modules to the introduction

Fixes #33796

Change-Id: I9f6837be96410e96d004523c48bef95ee1427484
Reviewed-on: https://go-review.googlesource.com/c/go/+/191746Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.13.html

@@ -31,7 +31,8 @@ Do not send CLs removing the interior tags from such phrases.
   for privacy information about these services and the
   <a href="/cmd/go/#hdr-Module_downloading_and_verification">go command documentation</a>
   for configuration details including how to disable the use of these servers or use
-  different ones.
+  different ones. If you depend on non-public modules, see the
+  <a href="/cmd/go/#hdr-Module_configuration_for_non_public_modules">documentation for configuring your environment</a>.
 </p>
 
 <h2 id="language">Changes to the language</h2>