avutil/pixfmt: Improve ascii art showing the meaning of the AVChromaLocation values

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/pixfmt.h

@@ -530,9 +530,13 @@ enum AVColorRange {
 /**
  * Location of chroma samples.
  *
- *  X   X      3 4 X      X are luma samples,
- *             1 2        1-6 are possible chroma positions
- *  X   X      5 6 X      0 is undefined/unknown position
+ * Illustration showing the location, the left shows only luma, the right
+ * shows the location of the chroma samples, the 2 could be imagined to overlay
+ * each other but are drawn seperately due to limitations of ASCII
+ *  ______        ______
+ * |X   X ...    |3 4 X ...     X are luma samples,
+ * |             |1 2           1-6 are possible chroma positions
+ * |X   X ...    |5 6 X ...     0 is undefined/unknown position
  */
 enum AVChromaLocation {
     AVCHROMA_LOC_UNSPECIFIED = 0,