Add documentation for window-tool-bar package

Add documentation for window-tool-bar package

Also change window-tool-mode to not pay attention to if
tool-bar-map has a buffer local value or not as that made the
documentation complicated.  Documentation added in Emacs manual,
Elisp manual, package commentary, and docstrings.  Also extend
window-tool-bar support to Emacs 27 and newer.  (bug#68765)

* doc/emacs/emacs.texi (Top):
* doc/emacs/frames.texi (Menu Bars):
* doc/emacs/glossary.texi (Glossary):
* doc/emacs/modes.texi (Minor Modes):
* doc/emacs/windows.texi (Windows, Tab Line): Mention Window
Tool Bar.
(Window Tool Bar): New documentation.
* doc/lispref/elisp.texi (Top):
* doc/lispref/modes.texi (Mode Line Format, Mode Line Basics)
(Mode Line Data): Mention Tab Lines.
(Tab Lines): New documentation.
* etc/NEWS: Mention newly added variable and package.
* lisp/window-tool-bar.el (window-tool-bar-mode): Don't display
tool bar when tool-bar-map is nil.
* lisp/window-tool-bar.el (tool-bar-always-show-default): Define
variable for older Emacs versions.