emacs: Improve interactive use documentation

emacs: Improve interactive use documentation

This improves the function documentation for many interactive
commands, either by improving their documentation string where the
improvement also makes sense for programmatic use or by adding a
'notmuch-doc property where it doesn't.

For nearly all commands that support a prefix argument, this adds a
'notmuch-prefix-doc property to document their prefixed behavior This
omits prefix documentation for a few commands where I thought the
prefixed behavior was too obscure (or too complex to fit in one line).