Doc: Abstract AF_UNIX sockets don't work on Windows.

An early release of AF_UNIX in Windows apparently supported Linux-style
"abstract" Unix sockets, but they do not seem to work in current Windows
versions and there is no mention of any of this in the Winsock
documentation.  Remove the mention of Windows from the documentation.

Back-patch to 14, where commit c9f0624b landed.

Discussion: https://postgr.es/m/CA%2BhUKGKrYbSZhrk4NGfoQGT_3LQS5pC5KNE1g0tvE_pPBZ7uew%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -793,7 +793,7 @@ include_dir 'conf.d'
        <para>
         A value that starts with <literal>@</literal> specifies that a
         Unix-domain socket in the abstract namespace should be created
-        (currently supported on Linux and Windows).  In that case, this value
+        (currently supported on Linux only).  In that case, this value
         does not specify a <quote>directory</quote> but a prefix from which
         the actual socket name is computed in the same manner as for the
         file-system namespace.  While the abstract socket name prefix can be