bug#67053: 29.1; Doc string of variable `dired-use-ls-dired'

[Drew Adams]

> Here are some thoughts to help explore a modification of the docstring.
> 
> I observe the rest of the docstring mentions MS Windows explicitly, but
> maybe incomplete for your suggestion.

Yes, it mentions MS Windows.  But that text is unrelated
to this bug, which is about the value that results from
checking for support of "--dired".  It's not clear from
the doc that that result can be `unspecified'.  The
wording suggests that the "check" clears up `unspecified',
replacing it, presumably, with either nil or some non-nil
value other than `unspecified'.

The special value of `unspecified' apparently does _not_
just mean that such a check is needed and will be done.
The value means that support isn't known, whether or
not the check has been done.