man: document the notmuch configuration settings in notmuch-config(1)

At the risk of duplication between the man page and the configuration
file generated by default, document the notmuch configuration options
in the notmuch config man page.

Existing users of notmuch should not be expected to re-generate their
config file in order to get access to the documentation for new
configuration options.

Include some minor fixes and cleanups while at it.