notmuch help: Simplify output by omitting arguments for each command.

notmuch help: Simplify output by omitting arguments for each command.

The output was far too busy otherwise. It's more useful to just
show the argument list in the case of "notmuch help <command>"
for a specific command.

(Credit due to running "git help" and seeing a much more readable
list than what was coming out of "notmuch help".)