avfilter/internal: Improve docs about ff_request_frame()

Reviewed-by: James Darnley <james.darnley@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/internal.h

@@ -305,8 +305,27 @@ int ff_poll_frame(AVFilterLink *link);
 /**
  * Request an input frame from the filter at the other end of the link.
  *
+ * The input filter may pass the request on to its inputs, fulfill the
+ * request from an internal buffer or any other means specific to its function.
+ *
+ * When the end of a stream is reached AVERROR_EOF is returned and no further
+ * frames.
+ *
+ * When a filter is unable to output a frame for example due to its sources
+ * being unable to do so or because it depends on external means pushing data
+ * into it then AVERROR(EAGAIN) is returned.
+ * It is important that a AVERROR(EAGAIN) return is returned all the way to the
+ * caller (generally eventually a user application) as this step may (but does
+ * not have to be) necessary to provide the input with the next frame.
+ *
+ * If a request is successful then the filter_frame() function will be called
+ * at least once before ff_request_frame() returns
+ *
  * @param link the input link
  * @return     zero on success
+ *             AVERROR_EOF on end of file
+ *             AVERROR(EAGAIN) if the previous filter cannot output a frame
+ *             currently and can neither guarantee that EOF has been reached.
  */
 int ff_request_frame(AVFilterLink *link);