git-describe.txt: document --always

Signed-off-by: NStephan Beyer <s-beyer@gmx.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -70,6 +70,9 @@ OPTIONS
 	Only consider tags matching the given pattern (can be used to avoid
 	leaking private tags made from the repository).
 
+--always::
+	Show uniquely abbreviated commit object as fallback.
+
 EXAMPLES
 --------