Documentation: clarify what is shown in "git-ls-files -s" output

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-files.txt

@@ -53,7 +53,7 @@ OPTIONS
 
 -s::
 --stage::
-	Show stage files in the output
+	Show staged contents' object name, mode bits and stage number in the output.
 
 --directory::
 	If a whole directory is classified as "other", show just its