Document scroll-up-line and scroll-down-line in Emacs manual.

Document scroll-up-line and scroll-down-line in Emacs manual.

* doc/emacs/display.texi (Scrolling): Document scroll-up-line and
scroll-down-line.  Document scroll-command property.
(Recentering): New node, split off from Scrolling.

Also, minor copyedits to standardize on the phrase "key binding"
rather than "keybinding" in the manual.