Add reference for status letters in documentation.

Also fix error in diff_filepair::status documentation, and point to
the in-code reference as well as the doc.
Signed-off-by: NYann Dirson <ydirson@altern.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/diff-format.txt

@@ -46,6 +46,22 @@ That is, from the left to the right:
 . path for "dst"; only exists for C or R.
 . an LF or a NUL when '-z' option is used, to terminate the record.
 
+Possible status letters are:
+
+- A: addition of a file
+- C: copy of a file into a new one
+- D: deletion of a file
+- M: modification of the contents or mode of a file
+- R: renaming of a file
+- T: change in the type of the file
+- U: file is unmerged (you must complete the merge before it can
+be committed)
+- X: "unknown" change type (most probably a bug, please report it)
+
+Status letters C and M are always followed by a score (denoting the
+percentage of similarity between the source and target of the move or
+copy), and are the only ones to be so.
+
 <sha1> is shown as all 0's if a file is new on the filesystem
 and it is out of sync with the index.
 

diffcore.h

@@ -58,7 +58,7 @@ struct diff_filepair {
 	struct diff_filespec *one;
 	struct diff_filespec *two;
 	unsigned short int score;
-	char status; /* M C R N D U (see Documentation/diff-format.txt) */
+	char status; /* M C R A D U etc. (see Documentation/diff-format.txt or DIFF_STATUS_* in diff.h) */
 	unsigned broken_pair : 1;
 	unsigned renamed_pair : 1;
 	unsigned is_unmerged : 1;