doc: mark `--output=summary` as default

`notmuch search` behaves differently depending on the output option: It
either outputs information pertaining to all threads with matching
messages (summary, threads) or to all matching messages (messages,
files, tags). The man page refres solely to the former in the main
description.

Help the user by clearly marking `summary` as the default output option.

Signed-off-by: Michael J Gruber <git@grubix.eu>