Document av_register_protocol(), not that it really needs to be

documented but it's nice to have everything documented.

Originally committed as revision 21471 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.h

@@ -226,6 +226,9 @@ URLProtocol *av_protocol_next(URLProtocol *p);
 attribute_deprecated int register_protocol(URLProtocol *protocol);
 #endif
 
+/**
+ * Registers the URLProtocol protocol.
+ */
 int av_register_protocol(URLProtocol *protocol);
 
 /**