Document CODEC_FLAG_EMU_EDGE and avcodec_align_dimensions interaction.

Originally committed as revision 23258 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3316,12 +3316,20 @@ unsigned avcodec_get_edge_width(void);
  * Modifies width and height values so that they will result in a memory
  * buffer that is acceptable for the codec if you do not use any horizontal
  * padding.
+ *
+ * May only be used if a codec with CODEC_CAP_DR1 has been opened.
+ * If CODEC_FLAG_EMU_EDGE is not set, the dimensions must have been increased
+ * according to avcodec_get_edge_width() before.
  */
 void avcodec_align_dimensions(AVCodecContext *s, int *width, int *height);
 /**
  * Modifies width and height values so that they will result in a memory
  * buffer that is acceptable for the codec if you also ensure that all
  * line sizes are a multiple of the respective linesize_align[i].
+ *
+ * May only be used if a codec with CODEC_CAP_DR1 has been opened.
+ * If CODEC_FLAG_EMU_EDGE is not set, the dimensions must have been increased
+ * according to avcodec_get_edge_width() before.
  */
 void avcodec_align_dimensions2(AVCodecContext *s, int *width, int *height,
                                int linesize_align[4]);