doc: mention godoc in "implements" FAQ

R=r
CC=golang-dev
https://golang.org/cl/4248066

doc/go_faq.html

@@ -558,7 +558,8 @@ type Fooer interface {
 
 <p>
 A type must then implement the <code>ImplementsFooer</code> method to be a
-<code>Fooer</code>, clearly documenting the fact.
+<code>Fooer</code>, clearly documenting the fact and announcing it in
+<a href="/cmd/godoc/">godoc</a>'s output.
 </p>
 
 <pre>