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).