Update Files chapter of Emacs manual.

Update Files chapter of Emacs manual.
Plus miscellaneous minor speling fixes in other manual files.

* doc/emacs/files.texi (File Names, Visiting, Interlocking): Copyedits.
(Backup Copying): backup-by-copying-when-mismatch is now t.
(Customize Save): Fix description of require-final-newline.
(Reverting): Note that revert-buffer can't be undone.  Mention VC.
(Auto Save Control): Clarify.
(File Archives): Add 7z.
(Remote Files): ange-ftp-make-backup-files defaults to nil.

* doc/emacs/arevert-xtra.texi (Autorevert): Copyedits.