doc: add fields enum for modify action in flow guide

doc: add fields enum for modify action in flow guide

Fix the documentation about the MODIFY_FIELD flow action.
1. Include the rte_flow_field_id enumeration reference to point
to the full list of all supported Field IDs available.
2. Correct the formatting of the MODIFY_FIELD action and the
destination/source field definition tables.

Fixes: 73b68f4c54a0 ("ethdev: introduce generic modify flow action")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>