Updated IPMI architecture documentation

This updates the documentation for the new IPMI architecture to reflect
what was actually implemented, which varied slightly from the
pie-in-the-sky dreams of what we wanted to start with. Some of it is
semantics, some of it is functional. But this should help in starting to
actually write new handlers based on the new architecture.

Change-Id: Iad39ac2de50f6b42384cad6baac9980c0e727b3f
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>