git-stash.txt: be explicit about subcommand options

git-stash.txt: be explicit about subcommand options

Currently, the options for the `list` and `show` subcommands are just
listed as `<options>`. This seems to imply, from a cursory glance at the
summary, that they take the stash options listed below. However, reading
more carefully, we see that they take log options and diff options
respectively.

Make it more obvious that they take log and diff options by explicitly
stating this in the subcommand summary.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>