effective_go.html: be more accepting in the guidelines for interface names

Fixes #5421.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/9274043

doc/effective_go.html

@@ -403,8 +403,10 @@ if owner != user {
 
 <p>
 By convention, one-method interfaces are named by
-the method name plus the -er suffix: <code>Reader</code>,
-<code>Writer</code>, <code>Formatter</code> etc.
+the method name plus an -er suffix or similar modification
+to construct an agent noun: <code>Reader</code>,
+<code>Writer</code>, <code>Formatter</code>,
+<code>CloseNotifier</code> etc.
 </p>
 
 <p>