Do not buttonize key bindings outside of *Help* buffers

Do not buttonize key bindings outside of *Help* buffers

* etc/NEWS: Mention the new variable.

* lisp/apropos.el (apropos-describe-plist): Bind the new
variable (bug#52053).
* lisp/button.el (button-describe): Bind the new variable.

* lisp/help-fns.el (describe-function, describe-variable)
(describe-face, describe-symbol, describe-syntax)
(describe-categories, describe-keymap, describe-mode)
(describe-widget): Bind the new variable.

* lisp/help-macro.el (make-help-screen): Bind the new variable.

* lisp/help.el (help-buffer-under-preparation): New variable
that is bound to t by commands that create a *Help* buffer.
(substitute-command-keys): Use the new variable:
help-link-key-to-documentation is supposed to have an effect
only "in *Help* buffers". Fixes bug#52053.
(view-lossage, describe-bindings, describe-key): Bind the new
variable.

* lisp/repeat.el (describe-repeat-maps): Bind the new variable.

* lisp/international/mule-cmds.el (describe-input-method)
(describe-language-environment): Bind the new variable.

* lisp/international/mule-diag.el (describe-character-set)
(describe-coding-system, describe-font, describe-fontset)
((list-fontsets): Bind the new variable.