switchdev: bring documentation up-to-date

Much need updated of switchdev documentation to cover what's been
implmented to-date.  There are some XXX comments in the text for
unimplemented or broken items.  I'd like to keep these in there (poor-man's
TODO list) and update the document once each issue is resolved.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>