Document platform-specificity of unix_socket_permissions.

Back-patch to 8.4 (all supported versions).

doc/src/sgml/config.sgml

@@ -640,7 +640,11 @@ include 'filename'
        </para>
 
        <para>
-        This parameter is irrelevant on Windows, which does not have
+        This parameter is irrelevant on systems, notably Solaris as of Solaris
+        10, that ignore socket permissions entirely.  There, one can achieve a
+        similar effect by pointing <varname>unix_socket_directories</> to a
+        directory having search permission limited to the desired audience.
+        This parameter is also irrelevant on Windows, which does not have
         Unix-domain sockets.
        </para>
       </listitem>