git-bundle.txt: Clarify rev-list-args restrictions

Currently, one could think that 'git bundle create' groks
any 'git rev-list' expression. But in fact it requires a named reference
to be present. Try and make this clearer.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bundle.txt

@@ -59,7 +59,8 @@ unbundle <file>::
 
 <git-rev-list-args>::
 	A list of arguments, acceptable to 'git rev-parse' and
-	'git rev-list', that specifies the specific objects and references
+	'git rev-list' (and containg a named ref, see SPECIFYING REFERENCES
+	below), that specifies the specific objects and references
 	to transport.  For example, `master{tilde}10..master` causes the
 	current master reference to be packaged along with all objects
 	added since its 10th ancestor commit.  There is no explicit