doc: announce ethdev port freeing on close operation

doc: announce ethdev port freeing on close operation

Since DPDK 18.11, some drivers are migrating to a new behaviour,
releasing port resources on close.
The temporary flag RTE_ETH_DEV_CLOSE_REMOVE triggers this new mode
in the migrated drivers.
After 2 years, the flag and the old behaviour will be removed.
Last drivers not migrated will have to complete the switch.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>