git-name-rev.txt: document --no-undefined and --always

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

Documentation/git-name-rev.txt

@@ -41,6 +41,13 @@ OPTIONS
 	of linkgit:git-describe[1] more closely.  This option
 	cannot be combined with --stdin.
 
+--no-undefined::
+	Die with error code != 0 when a reference is undefined,
+	instead of printing `undefined`.
+
+--always::
+	Show uniquely abbreviated commit object as fallback.
+
 EXAMPLE
 -------