Double the default value of kill-ring-max

Double the default value of kill-ring-max

* lisp/simple.el (kill-ring-max): Double the default to 120.
* lisp/menu-bar.el (yank-menu-length): Doc fix.
(yank-menu-max-items): New variable.
(menu-bar-update-yank-menu): Don't display more than
'yank-menu-max-items' in the yank menu.

* doc/emacs/custom.texi (Changing a Variable):
* doc/emacs/killing.texi (Kill Ring):
* doc/lispintro/emacs-lisp-intro.texi (kill-new function):
* doc/lispref/text.texi (Internals of Kill Ring): Doc fix to use
the new value.