Mention that SSPI authentication can use GSSAPI on non-Windows systems.

As noted by Christian Ullrich.

doc/src/sgml/client-auth.sgml

@@ -990,7 +990,9 @@ omicron         bryanh                  guest1
     <productname>Kerberos</productname> when possible and automatically
     fall back to <productname>NTLM</productname> in other cases.
     <productname>SSPI</productname> authentication only works when both
-    server and client are running <productname>Windows</productname>.
+    server and client are running <productname>Windows</productname>,
+    or, on non-Windows platforms, when <productname>GSSAPI</productname>
+    is available.
    </para>
 
    <para>