vswitch.xml: Improve IPFIX documentation.

vswitch.xml: Improve IPFIX documentation.

A coworker came by yesterday with a question about IPFIX in Open vSwitch.
I did not know the answer, so I read the documentation.  The documentation
was harder to understand than necessary.  This change should make it easier
to understand by providing an overview of the two forms of configuration
and grouping together the columns that apply only to one form of
configuration.

Signed-off-by: Ben Pfaff <blp@nicira.com>