emacs: `notmuch-mua-new-forward-message' is not interactive
`notmuch-mua-new-forward-message' must be called from a buffer
containing a raw RFC2822-formatted message to forward. Hence, it's
intended to be invoked programmatically through something else that
sets up this buffer (like `notmuch-show-forward-message'), not
interactively.
Remove its interactive specification and update the documentation
string to mention the requirements on the current buffer.