Automatically document when setopt is needed

Automatically document when setopt is needed

* lisp/help-fns.el (help--recommend-setopt): New function to
automatically document the need to use `setopt` to set the values of
any defcustoms with a `:set` property.
(help-fns-describe-variable-functions): Add above new function to
hook.

* lisp/follow.el (follow-mode-prefix-key):
* lisp/minibuffer.el (minibuffer-regexp-prompts):
* lisp/register.el (register-use-preview):
* lisp/savehist.el (savehist-autosave-interval):
* lisp/saveplace.el (save-place-autosave-interval):
* lisp/tab-bar.el (tab-bar-define-keys):
* lisp/textmodes/text-mode.el (text-mode-ispell-word-completion):
Delete now redundant text from docstrings.

(cherry picked from commit 77b7e2d37b2c68b73ad770ec680281c1f82d26f1)