bug#62126: 26.3; Doc of `dired-do(-async)-shell-command': mention `dired-guess-shell-command'

[Drew Adams]

It would help if the doc of these commands (and others that use
`dired-read-shell-command') mentioned that if `dired-x' is loaded then
they make use of option `dired-guess-shell-command' to guess an
appropriate default value (command).

To find this out now, users need to check the implementations of such
commands, see that they call `dired-read-shell-command', and check the
doc of that function.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19044
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''