bug#43868: 27.1.50; Documentation: Mention that minor mode hooks are also run when the minor mode is deactivated

[Eli Zaretskii <eliz@gnu.org>]

> From: Joost Kremers <joostkremers@fastmail.fm>
> Date: Thu, 08 Oct 2020 20:46:50 +0200
> 
> Minor mode hooks are run when the minor mode is activated but also when
> the mode is deactivated. This is mentioned in the default doc string for minor
> mode hooks, but it does not seem to be mentioned in the Elisp manual.
> 
> I checked the section "Modes > Minor Modes > Defining Minor Modes", which
> mentions that `define-minor-mode` creates a hook variable, but it doesn't
> mention when it is run. I think it should be mentioned that it is run both when
> the mode is activated and when the mode is deactivated.
> 
> (Other sections don't seem to be as relevant: while searching for information on
> this, I also looked in the section "Customization > Variables > Hooks", but that
> section doesn't mention minor mode hooks at all. Same thing for the Emacs
> manual: it doesn't seem to mention minor mode hooks anywhere.)

"Minor Mode Conventions"

     In addition, several major mode conventions (*note Major Mode
  Conventions::) apply to minor modes as well: those regarding the names
  of global symbols, the use of a hook at the end of the initialization
  function, and the use of keymaps and other tables.

"Major Mode Conventions":

     • Each major mode should have a normal “mode hook” named
       ‘MODENAME-mode-hook’.  The very last thing the major mode command
       should do is to call ‘run-mode-hooks’.  This runs the normal hook
       ‘change-major-mode-after-body-hook’, the mode hook, the function
       ‘hack-local-variables’ (when the buffer is visiting a file), and
       then the normal hook ‘after-change-major-mode-hook’.  *Note Mode
       Hooks::.

"Mode Hooks":

  Every major mode command should finish by running the mode-independent
  normal hook ‘change-major-mode-after-body-hook’, its mode hook, and the
  normal hook ‘after-change-major-mode-hook’.  It does this by calling
  ‘run-mode-hooks’.

Since the mode command runs both when turning the mode on and when
turning it off, the above implies that the mode hook runs in both
situations.