emacs: Clarify that notmuch-poll-script is deprecated

notmuch-poll-script has long since been deprecated in favor of
post-new hooks, but this wasn't obvious from the documentation.
Update the documentation to make this clear.  Since
notmuch-poll-script could, to some extend, be used to control the path
of the notmuch binary and that use is now clearly discouraged, promote
notmuch-command to a real defcustom instead of just a variable.