emacs: add support for reversing notmuch-show-mark-read tag changes

Since marking a message as read can now be a user customized set of
tag changes, make reversing this easier. Allow a prefix argument to
notmuch-show-mark-read to reverse the marking as read, similar to the
unarchiving in notmuch-show-archive-message.

While at it, update the relevant documentation to match that of other
automatic tagging (i.e. archive and reply).