Documentation: Add documentation for new devlink-rate attributes

Provide documentation for newly introduced netlink attributes for
devlink-rate: tx_priority and tx_weight.

Mention the possibility to export tree from the driver.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>