conf: describe security_driver behavior

As a request was raised for this, I added few lines in the "Notes"
part of the "security_driver" comments about allowed values.

src/qemu/qemu.conf

@@ -164,7 +164,10 @@
 #
 #       security_driver = [ "selinux", "apparmor" ]
 #
-# Note: The DAC security driver is always enabled.
+# Notes: The DAC security driver is always enabled; as a result, the
+# value of security_driver cannot contain "dac".  The value "none" is
+# a special value; security_driver can be set to that value in
+# isolation, but it cannot appear in a list of drivers.
 #
 #security_driver = "selinux"