Clarify doxy for av_fifo_alloc().

Originally committed as revision 22812 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/fifo.h

@@ -35,7 +35,7 @@ typedef struct AVFifoBuffer {
 /**
  * Initializes an AVFifoBuffer.
  * @param size of FIFO
- * @return AVFifoBuffer or NULL if mem allocation failure
+ * @return AVFifoBuffer or NULL in case of memory allocation failure
  */
 AVFifoBuffer *av_fifo_alloc(unsigned int size);