Restore follow-scroll-up/down to scrolling by the combined size of all windows
Also rename the current follow-scroll-up/down functions to
follow-scroll-up-window and follow-scroll-down-window. These scroll by the
height of the current window.
This fixes bug #23347.
* lisp/follow.el (follow-mode): Tweak the doc string.
(follow-scroll-up-arg, follow-scroll-down-arg): new functions, extracted from
follow-scroll-up/down.
(follow-scroll-up-window, follow-scroll-down-window): Functions renamed from
follow-scroll-up/down.
(follow-scroll-up, follow-scroll-down): Restore the historic functionality.