Fixes #5421.
R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/
9274043
<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>