iflibtxrx.9: update function descriptions to match implementation

isc_rxd_refill, isc_rxd_flush return nothing, not void *.

isc_txd_credits_update, isc_rxd_available return int, not int *.

isc_txd_credits_update has a bool as final argument, not a uint32_t.
Prior to r315217 it took four arguments; the final two were
uint32_t, bool.

Reported by: Gerald Aryeetey <aryeeteygerald_rogers.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation