doc: clarify that contributing isn't just for code

Updates #33323

Change-Id: I4b3076469f3db8ab0fe888edcf0471f17dc199e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/202823Reviewed-by: Robert Griesemer <gri@golang.org>

doc/contrib.html

@@ -115,7 +115,7 @@ Community-related issues should be reported to
 See the <a href="/conduct">Code of Conduct</a> for more details.
 </p>
 
-<h3><a href="/doc/contribute.html">Contributing code</a></h3>
+<h3><a href="/doc/contribute.html">Contributing code &amp; documentation</a></h3>
 
 <p>
 Go is an open source project and we welcome contributions from the community.