* lisp/emacs-lisp/generic.el (define-generic-mode): Say what a
generic mode is (bug#43713).
function-list &optional docstring)
"Create a new generic mode MODE.
+A \"generic\" mode is a simple major mode with basic support for
+comment syntax and Font Lock mode, but otherwise do not have a
+any special keystrokes or functionality available.
+
MODE is the name of the command for the generic mode; don't quote it.
The optional DOCSTRING is the documentation for the mode command. If
you do not supply it, `define-generic-mode' uses a default