git-show-ref.txt: clarify the pattern matching

git-show-ref really does not do what one would expect under the name
pattern matching, so describe it.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-ref.txt

@@ -84,7 +84,11 @@ OPTIONS
 
 <pattern>...::
 
-	Show references matching one or more patterns.
+	Show references matching one or more patterns. Patterns are matched from
+	the end of the full name, and only complete parts are matched, e.g.
+	'master' matches 'refs/heads/master', 'refs/remotes/origin/master',
+	'refs/tags/jedi/master' but not 'refs/heads/mymaster' nor
+	'refs/remotes/master/jedi'.
 
 OUTPUT
 ------