bug#13226: 24.3.50; set-file-acl on MS Windows shall check ACL string format

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: michael.albinus@gmx.de,  romain@orebokech.com,  13226@debbugs.gnu.org
> Date: Wed, 19 Dec 2012 19:53:08 -0500
> 
> > Currently, it could be either a string or nil.  The latter means "no
> > ACL".  Do you want to undocument both of these possibilities?  It
> > sounds too radical to me.
> 
> Of course nil is perfectly fine, but the string one shouldn't
> be documented (and ideally, the it shouldn't be a string but an opaque
> object).

Done (the documentation part) in trunk revision 111296.