git-fast-export.txt: clarify misleading documentation about rev-list args

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-export.txt

@@ -119,7 +119,8 @@ marks the same across runs.
 	'git rev-list', that specifies the specific objects and references
 	to export.  For example, `master~10..master` causes the
 	current master reference to be exported along with all objects
-	added since its 10th ancestor commit.
+	added since its 10th ancestor commit and all files common to
+	master{tilde}9 and master{tilde}10.
 
 EXAMPLES
 --------