Documentation should mention impact of single-layer-serialization on OpenFlow protocol support

It appears that single layer serialization has an impact on OpenFlow protocol support through the external API. In particular, the OFPMC_MODIFY command does not appear to be supported when single layer serialization is enabled, while it is supported when multi-layer serialization is enabled. It is possible that other messages beyond ofp_meter_mod messages are also affected, although I did not test those. I don't believe this is documented anywhere.

At minimum, I believe that the documentation for the use-single-layer-serialization configuration option in org.opendaylight.openflowplugin.cfg should make it clear that use of single layer serialization has some functional impact on OpenFlow protocol support. Additional documentation in the OpenFlow plugin users guide in the form of a note would also be useful.