Make Gnus FAQ more contemporary

Make Gnus FAQ more contemporary

* doc/misc/gnus-faq.texi (FAQ - Introduction): Fix timeline.
(FAQ - Changes): Delete outdated section.
(FAQ 5-5): Update to reflect that ispell.el and flyspell.el is
distributed with Emacs.
(FAQ 4-1, FAQ 4-7, FAQ 4-8, FAQ 4-14, FAQ 5-8, FAQ 5-10, FAQ 6-3)
(FAQ 7-2): Delete references to ancient versions of Gnus.
(FAQ 6-3): Delete reference to obsolete library nnir.el.
(FAQ - Glossary): Remove "Emacs" entry.  This is self-explanatory, and
risks causing confusion; our docs never mean anything but "GNU Emacs"
when we say "Emacs".