bug#33108: Expose hidden "Save for Future Sessions" Customizations button

[積丹尼 Dan Jacobson]

In the Customize Face buffer most users will think the "Apply and Save"
button will save their changes to a file, only to unknowingly lose them
upon exit.

Only if they read (info "(emacs) Saving Customizations") will they learn
of the hidden "Save for Future Sessions" button.

I propose the latter button be placed on the interface right beside the
former button, visible from the start.

Also whatever makes this comment
 ;; custom-set-... was added by Custom.
 ;; If you edit it by hand, you could mess it up, so be careful.
 ;; Your init file should contain only one such instance.
 ;; If there is more than one, they won't work right.
should mention it too.