Clarify docstring promise to only apply to unsafe quoting.

Shell quoting does not apply to entire argv elements, but only
individual characters thereof -- and different characters within a
string can be quoted in different ways. As such, a promise to treat
"pre-quoted" argv elements differently is not sensible; moreover, this
promise restricts the set of data which can be legitimately passed
(whereas natively, argv array entries can contain any non-NUL bytes --
including, of course, embedded literal quotes).

The new, safe algorithm does not respect the traditional
implementation's promise to preserve original quoting. Clarify this in
the docstring.