[media] DocBook: document typedef dmx_section_cb at demux.h

The dvb/kdapi.tmpl has already an extensive documentation about
this callback. Now that we've added function typedefs at kernel-doc,
add such documentation at demux.h, for it to appear at device-drivers
DocBook.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-core/demux.h

@@ -256,6 +256,41 @@ typedef int (*dmx_ts_cb)(const u8 *buffer1,
 			 size_t buffer2_length,
 			 struct dmx_ts_feed *source);
 
+/**
+ * typedef dmx_section_cb - DVB demux TS filter callback function prototype
+ *
+ * @buffer1:		Pointer to the start of the filtered section, e.g.
+ *			within the circular buffer of the demux driver.
+ * @buffer1_len:	Length of the filtered section data in @buffer1,
+ *			including headers and CRC.
+ * @buffer2:		Pointer to the tail of the filtered section data,
+ *			or NULL. Useful to handle the wrapping of a
+ *			circular buffer.
+ * @buffer2_len:	Length of the filtered section data in @buffer2,
+ *			including headers and CRC.
+ * @source:		Indicates which section feed is the source of the
+ *			callback.
+ *
+ * This function callback prototype, provided by the client of the demux API,
+ * is called from the demux code. The function is only called when
+ * filtering of sections has been enabled using the function
+ * &dmx_ts_feed.@start_filtering. When the demux driver has received a
+ * complete section that matches at least one section filter, the client
+ * is notified via this callback function. Normally this function is called
+ * for each received section; however, it is also possible to deliver
+ * multiple sections with one callback, for example when the system load
+ * is high. If an error occurs while receiving a section, this
+ * function should be called with the corresponding error type set in the
+ * success field, whether or not there is data to deliver. The Section Feed
+ * implementation should maintain a circular buffer for received sections.
+ * However, this is not necessary if the Section Feed API is implemented as
+ * a client of the TS Feed API, because the TS Feed implementation then
+ * buffers the received data. The size of the circular buffer can be
+ * configured using the &dmx_ts_feed.@set function in the Section Feed API.
+ * If there is no room in the circular buffer when a new section is received,
+ * the section must be discarded. If this happens, the value of the success
+ * parameter should be DMX_OVERRUN_ERROR on the next callback.
+ */
 typedef int (*dmx_section_cb)(const u8 *buffer1,
 			      size_t buffer1_len,
 			      const u8 *buffer2,