doc: announce ethdev functions will not return void

doc: announce ethdev functions will not return void

void return value is bad for get API (like rte_eth_dev_info-get())
since caller does not know if the function does its job or not and
output value is filled in.

void return value is bad for state changing API (like
rte_eth_promiscuous_enable()) since caller should use get API
to understand if state is really changed.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Jerin Jacob <jerinj@marvell.com>