Mention in the documentation of avfilter_poll_frame() that it returns

a negative value in case of error.

Originally committed as revision 16372 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/avfilter.h

@@ -509,7 +509,8 @@ int avfilter_request_frame(AVFilterLink *link);
 /**
  * Poll a frame from the filter chain.
  * @param  link the input link
- * @return      the number of imediately available frames
+ * @return the number of immediately available frames, a negative
+ * number in case of error
  */
 int avfilter_poll_frame(AVFilterLink *link);