Improve Lisp-level documentation of tooltips

Improve Lisp-level documentation of tooltips

* doc/lispref/display.texi (Tooltips): New section.  (Bug#23246)
(Display): Update the chapter menu.
* doc/lispref/text.texi (Special Properties): Make the "tooltip"
index entry more concrete.  Change the cross-reference to point to
"Tooltips" in the ELisp manual.
* doc/lispref/elisp.texi (Top): Update the master menu.
* doc/emacs/frames.texi (Tooltips): Include more customization
variables.  Add a cross-reference to the ELisp manual.