Reorganise (auto-)revert nodes in the manual

Reorganise (auto-)revert nodes in the manual

Put all information about auto-revert into a section of its own, and
organise the text in a more logical way.  Previously it was mainly
described in the section about reverting (bug#35418).

* doc/emacs/files.texi (Files): Adjust menu.
(Reverting, Auto Revert, Autorevert): Add node `Auto Revert' and move
text on that topic from `Reverting', rearranged.  Turn the old
`Autorevert' node into a subsection under `Auto Revert'.
* doc/emacs/arevert-xtra.texi (Autorevert): Rename and turn into
subsubsection.
* doc/emacs/buffers.texi (Several Buffers): Adjust references.
* doc/emacs/emacs.texi (Top): Adjust menu.
* doc/emacs/emacs-xtra.texi (Top): Adjust menu.
(Non-File Buffers): Add node and section lines.