bug#16562: dired-do-shell-command's docstring should mention where the suggestions come from

[Drew Adams]

> dired-do-shell-command's docstring should mention
> dired-guess-shell-alist-default and
> dired-guess-shell-alist-user.

At the risk of giving this worthwhile suggestion the kiss of
death: +1.

These variables should probably be mentioned also in at least
one of the Emacs manuals.

The latter variable provides a way for users to control/customize
file-program associations, which is pretty important.  As it
stands now, users will know about this possibility only from
Emacs Wiki, diverse Web forums, or by stumbling across it in the
source code.  As a result, I doubt that many Emacs users are
aware of these variables.

My guess is that the lack of doc for this is a side effect
of the fact that it is a Dired-X feature.  And so far, Dired-X
is still treated as a poor step-child (as is also Dired Aux).