describe: Document --match pattern format

It's not clear in git-describe(1) what kind of "pattern" should be
passed to --match.  Fix that.

Signed-off-by: Greg Price <price@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -81,8 +81,9 @@ OPTIONS
 	that points at object deadbee....).
 
 --match <pattern>::
-	Only consider tags matching the given pattern (can be used to avoid
-	leaking private tags made from the repository).
+	Only consider tags matching the given `glob(7)` pattern,
+	excluding the "refs/tags/" prefix.  This can be used to avoid
+	leaking private tags from the repository.
 
 --always::
 	Show uniquely abbreviated commit object as fallback.