notmuch help: Be less verbose by default and support detailed help

Putting all of our documentation into a single help message was getting
a bit unwieldy. Now, the simple output of "notmuch help" is a reasonable
reminder and a quick reference. Then we now support a new syntax of:
"notmuch help <command>" for the more detailed help messages.

This gives us freedom to put more detailed caveats, etc. into some
sub-commands without worrying about the usage statement getting too
long.