doc: clarify default go command behavior in the release notes

Change-Id: I2a0bd295ad9737581d149a7165191b878ae2acda
Reviewed-on: https://go-review.googlesource.com/c/go/+/181880Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.13.html

@@ -25,12 +25,13 @@ Do not send CLs removing the interior tags from such phrases.
 </p>
 
 <p>
-  As of Go 1.13, the go command downloads and authenticates modules using
-  the Go module mirror and Go checksum database run by Google. See
+  As of Go 1.13, the go command by default downloads and authenticates
+  modules using the Go module mirror and Go checksum database run by Google. See
   <a href="https://proxy.golang.org/privacy">https://proxy.golang.org/privacy</a>
   for privacy information about these services and the
   <a href="/cmd/go/#hdr-Module_downloading_and_verification">go command documentation</a>
-  for configuration details.
+  for configuration details including how to disable the use of these servers or use
+  different ones.
 </p>
 
 <p>