diff-format doc: a score can follow M for rewrite

b6d8f30 (diff-raw format update take #2., 2005-05-23) started
documenting the diff format, and it said

 ...
 (8) sha1 for "dst"; 0{40} if creation, unmerged or "look at work tree".
 (9) status, followed by similarlity index number only for C and R.
 (10) a tab or a NUL when '-z' option is used.
 ...

because C and R _were_ the only ones that came with a number back
then.  This was corrected by ddafa7e (diff-helper: Fix R/C score
parsing under -z flag., 2005-05-29) and we started saying "score"
instead of "similarlity index" (because we can have other kind of
score there), and stopped saying "only for C and R" (because Git is
an ever evolving system).  Later f345b0a (Add -B flag to diff-*
brothers., 2005-05-30) introduced a new concept, "dissimilarity"
score; it did not have to fix any documentation.

The current text that says only C and R can have scores came
independently from a5a323f (Add reference for status letters in
documentation., 2008-11-02) and it was wrong from the day one.

Noticed-by: Mike Hommey
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-format.txt

@@ -66,7 +66,8 @@ be committed)
 
 Status letters C and R 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.
+copy).  Status letter M may be followed by a score (denoting the
+percentage of dissimilarity) for file rewrites.
 
 <sha1> is shown as all 0's if a file is new on the filesystem
 and it is out of sync with the index.