mention avcodec_align_dimensions()

Originally committed as revision 3136 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -897,7 +897,8 @@ typedef struct AVCodecContext {
     /**
      * called at the beginning of each frame to get a buffer for it.
      * if pic.reference is set then the frame will be read later by lavc
-     * width and height should be rounded up to the next multiple of 16
+     * avcodec_align_dimensions() should be used to find the required width and
+     * height, as they normally need to be rounded up to the next multiple of 16
      * - encoding: unused
      * - decoding: set by lavc, user can override
      */