Improve wording and markup in gnus-faq.texi

Improve wording and markup in gnus-faq.texi

* doc/misc/gnus-faq.texi (FAQ 2-4): Improve wording and markup.
(FAQ 2-1, FAQ 2-4, FAQ 2-5, FAQ 3 - Getting Messages, FAQ 3-1)
(FAQ 3-2, FAQ 3-2, FAQ 3-4, FAQ 4-1, FAQ 4-2, FAQ 4-3, FAQ 4-4)
(FAQ 4-8, FAQ 4-9, FAQ 4-10, FAQ 4-12, FAQ 5-1, FAQ 5-2)
(FAQ 5-7, FAQ 6-1, FAQ 6-2, FAQ 6-2, FAQ 6-3, FAQ 6-4, FAQ 6-5)
(FAQ 7-2, FAQ 7-3, FAQ 7-3, FAQ 7-4, FAQ 8-1, FAQ 8-5, FAQ 9-1):
Improve markup and reflow some paragraphs.