bug#24706: 26.0.50; Minor mode functions should do strict argument type checking

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sun, 16 Oct 2016 18:25:08 +0000
> 
> Attached a patch that uses the wording from `define-minor-mode'. 

The patch for the ELisp manual simply rearranges the same words, so
it's not clear to me why we would prefer it to what's already there.

As for the doc string, please avoid repetition, it's confusing.  I
suggested to describe the additional features when the mode is called
from Lisp by using the word "also".

Thanks.