bug#37771: 26.3; doc for `dired-auto-revert-buffer'

[Drew Adams]

Please consider improving the doc so that it does the following:

* Explains what is meant by "visiting" a Dired buffer.  The Emacs
  manuals and other doc only speak about visiting a file in a buffer.
  Just what constitutes "visiting" a Dired buffer?  It seems like it's
  sufficient to display the buffer and select its window.  Is that it?

* Explains the Value Menu items in Customize - in particular, item
  `Revert changed Dired buffer'.

* Explains that the option has no effect on Dired buffers containing
  arbitrary file lists (i.e., whose DIRNAME is a cons).  It is relevant
  only for actual directory listings.

* Lists the Lisp values that correspond to the Value Menu items.


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.17763
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''