bug#46239: 26.3; doc string of `dired-mode' wrt "Customization variables"

[Drew Adams]

Just noticed that vanilla `dired-mode' says this:

 Customization variables (rename this buffer and type C-h v on each line
 for more info):

   `dired-listing-switches'
   `dired-trivial-filenames'
   `dired-marker-char'
   `dired-del-marker'
   `dired-keep-marker-rename'
   `dired-keep-marker-copy'
   `dired-keep-marker-hardlink'
   `dired-keep-marker-symlink'

These two variables are NOT user options, which is usually what Emacs
means nowadays when speaking of "customization variables":
`dired-marker-char' and `dired-del-marker'.  That characterization here
is a bit misleading.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''