doc: announce removal of old port count function

doc: announce removal of old port count function

The function rte_eth_dev_count() was marked as deprecated in DPDK 18.05
in commit d9a42a69febf ("ethdev: deprecate port count function").
It is planned to be removed after the next LTS release.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>