ionic: update documentation for XDP support

Add information to our documentation for the XDP features
and related ethtool stats.

While we're here, we also add the missing timestamp stats.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240319163534.38796-1-shannon.nelson@amd.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>