vswitch.xml: Improve documentation for BFD.

vswitch.xml: Improve documentation for BFD.

This update improves the BFD documentation in a few ways:

   - Demand mode is now supported.

   - Wordsmithing, spelling, etc.

   - Attempt to better explain decay_min_rx, forwarding_if_rx, and
     cpath_down.

   - Break into subgroups for configuration and status, to better explain
     which party sets which fields.

   - Reindents to match the rest of vswitch.xml.

Because of the reindentation, this patch may be easier to view with spacing
changes suppressed.

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