Improve the Emacs manual's "Misc" node

Improve the Emacs manual's "Misc" node

* doc/emacs/misc.texi (Gnus): A more descriptive section name.
(Invoking emacsclient): Mention how to avoid errors when there's
no Emacs server.
(EWW, Embedded WebKit Widgets): Now subsections under
"Hyperlinking and Web Navigation Features".
(Amusements): Add "games" to the section name.  Suggested by Alex
Branham <alex.branham@gmail.com> in emacs-manual-bugs@gnu.org.

* doc/emacs/emacs.texi (Top): Adjust the top-level menus to the
above changes.

* doc/lispref/os.texi (Security Considerations): Fix typos.