Updates to misc.texi in Emacs manual.

Updates to misc.texi in Emacs manual.

* misc.texi (Gnus, Buffers of Gnus): Copyedits.
(Gnus Startup): Note that the system might not be set up for news.
Describe group levels more clearly.
(Gnus Group Buffer, Gnus Summary Buffer): New nodes, split from
Summary of Gnus.
(Document View): Copyedits.  Move zoom commads to DocView
Navigation node.
(DocView Navigation, DocView Searching, DocView Slicing)
(DocView Conversion): Nodes renamed from Navigation, etc.

* sending.texi (Mail Sending): Add message-kill-buffer-on-exit.