diff-format.txt: dst can be 0* SHA-1 when path is deleted, too

"dst" can legitimately be "0\{40\}" for a creation patch, e.g. when
the stat information is stale, but it falls into "look at work tree"
case.  The original description in b6d8f30 ([PATCH] diff-raw format
update take microsoft#2., 2005-05-23) forgot that deletion also makes the
"dst" 0* SHA-1.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-format.txt

@@ -43,7 +43,7 @@ That is, from the left to the right:
 . a space.
 . sha1 for "src"; 0\{40\} if creation or unmerged.
 . a space.
-. sha1 for "dst"; 0\{40\} if creation, unmerged or "look at work tree".
+. sha1 for "dst"; 0\{40\} if deletion, unmerged or "look at work tree".
 . a space.
 . status, followed by optional "score" number.
 . a tab or a NUL when `-z` option is used.