Clarify behavior of initdb's --allow-group-access on Windows in docs

The option is ignored on Windows, and GUC data_directory_mode already
mentioned that within its description in the documentation.

Author: Michael Paquier
Reported-by: Haribabu Kommi, David Steele
Discussion: https://postgr.es/m/CAJrrPGefxTG43yk6BrOC7ZcMnCTccG9+inCSncvyys_t8Ev9cQ@mail.gmail.com
Backpatch-through: 11

doc/src/sgml/ref/initdb.sgml

@@ -202,7 +202,9 @@ PostgreSQL documentation
       <listitem>
        <para>
         Allows users in the same group as the cluster owner to read all cluster
-        files created by <command>initdb</command>.
+        files created by <command>initdb</command>.  This option is ignored
+        on <productname>Windows</productname> as it does not support
+        <acronym>POSIX</acronym>-style group permissions.
        </para>
       </listitem>
      </varlistentry>