ethdev: add doxygen comment for Rx timestamp API

ethdev: add doxygen comment for Rx timestamp API

The offload flag DEV_RX_OFFLOAD_TIMESTAMP had no documentation.
After switching to dynamic mbuf flag and field,
it becomes even more important to explicit the feature behaviour.

A doxygen comment for the timesync API was mentioning
the deprecated timestamp field, so it is also updated.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Olivier Matz <olivier.matz@6wind.com>