RDMA/efa: Device definitions documentation updates

Various clarifications and updates to the documentation of the device
definitions.
No functional changes in this patch.

Link: https://lore.kernel.org/r/20200114085706.82229-4-galpress@amazon.com


Reviewed-by: Firas JahJah <firasj@amazon.com>
Reviewed-by: Yossi Leybovich <sleybo@amazon.com>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>