doc/go1.1.html: document io.ErrNoProgress

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8833050

doc/go1.1.html

@@ -832,6 +832,8 @@ reads progressive JPEG files and handles a few more subsampling configurations.
 The <a href="/pkg/io/"><code>io</code></a> package now exports the
 <a href="/pkg/io/#ByteWriter"><code>io.ByteWriter</code></a> interface to capture the common
 functionality of writing a byte at a time.
+It also exports a new error, <a href="/pkg/io/#ErrNoProgress"><code>ErrNoProgress</code></a>,
+used to indicate a <code>Read</code> implementation is looping without delivering data.
 </li>
 
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