Extend doxy for the src parameter of av_fifo_generic_write().

@patchby Tomas Härdin |tomas dot hardin at codemill dot se|

Originally committed as revision 22043 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/fifo.h

@@ -81,7 +81,8 @@ int av_fifo_generic_read(AVFifoBuffer *f, void *dest, int buf_size, void (*func)
 /**
  * Feeds data from a user-supplied callback to an AVFifoBuffer.
  * @param *f AVFifoBuffer to write to
- * @param *src data source
+ * @param *src data source; non-const since it may be used as a
+ * modifiable context by the function defined in func
  * @param size number of bytes to write
  * @param *func generic write function; the first parameter is src,
  * the second is dest_buf, the third is dest_buf_size.