Document some Emacs 24 scrolling changes.

Document some Emacs 24 scrolling changes.

* doc/emacs/basic.texi (Moving Point):
* doc/emacs/custom.texi (Mouse Buttons):
* doc/emacs/rmail.texi (Rmail Scrolling):
* doc/emacs/search.texi (Isearch Scroll):

* doc/emacs/display.texi (Scrolling): Replace scroll-up/down with
scroll-up/down-command.  Fix scroll-preserve-screen-position
description.  Document scroll-error-top-bottom.

* doc/lispref/windows.texi (Textual Scrolling): Document scroll-up-command,
scroll-down-command, scroll-error-top-bottom, and the
scroll-command symbol property.