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 <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>
Acked-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@redhat.com>
Acked-by: Andrew Rybchenko <4155e7bf8e0185127625dac6d84336299844982f@oktetlabs.ru>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk