doc: Mention godoc's handling of example functions.

Fixes #4625.

R=iant, adg
CC=golang-dev
https://golang.org/cl/7064052

doc/articles/godoc_documenting_go_code.html

@@ -137,3 +137,9 @@ indexing via the <code>-path</code> flag or just by running <code>"godoc ."</cod
 in the source directory. See the <a href="/cmd/godoc/">godoc documentation</a>
 for more details.
 </p>
+
+<p>
+Godoc recognizes example functions written according to the
+<a href="/pkg/testing/#pkg-overview"><code>testing</code></a> package's naming
+conventions and presents them appropriately.
+</p>