Updates to Display chapter of Lisp manual.

Updates to Display chapter of Lisp manual.

* doc/lispref/display.texi (Fringes): Note that fringes are shown on graphical
displays only.
(Fringe Size/Pos, Fringe Bitmaps, Making Buttons): Clarifications.
(Replacing Specs): Clarify example.
(Manipulating Buttons): Note that button-at can return a marker.
(Buttons): Minor rewrite.
(Character Display): New node.  Consolidate all character display
related nodes into its subsections.
(Usual Display): Character 127 is also affected by ctl-arrow.
(Display Tables): Improve example.

* lisp/button.el (button-at): Minor addition to docstring.