Document order of preference for format selection (related #798)

documentation.html

@@ -96,7 +96,7 @@ For a complete list, execute <code>youtube-dl --list-extractors</code>.
 
 <h1 id="d6">YouTube formats</h1>
 
-<p>Using the -f option and other related options, you can specify the video format to be downloaded from YouTube. Instead of keeping a video format table here, I will refer you to the <a href="http://en.wikipedia.org/wiki/YouTube#Quality_and_codecs">list of YouTube formats on Wikipedia</a>.</p>
+<p>Using the -f option and other related options, you can specify the video format to be downloaded from YouTube. If you have an order of preference, specify the formats separating them with slashes: <code>-f 22/17/18</code>. Instead of keeping a video format table here, I will refer you to the <a href="http://en.wikipedia.org/wiki/YouTube#Quality_and_codecs">list of YouTube formats on Wikipedia</a>.</p>
 
 <h1 id="d7">Output template</h1>