MFC r272730,273018:
  Add support for QAC AR816x/AR817x Gigabit/Fast Ethernet controllers.
  These controllers seem to have the same feature of AR813x/AR815x and
  improved RSS support(4 TX queues and 8 RX queues).  alc(4) supports
  all hardware features except RSS.  I didn't implement RX checksum
  offloading for AR816x/AR817x just because I couldn't get
  confirmation from the Vendor whether AR816x/AR817x corrected its
  predecessor's RX checksum offloading bug on fragmented packets.
  This change adds supports for the following controllers.
   o AR8161 PCIe Gigabit Ethernet controller
   o AR8162 PCIe Fast Ethernet controller
   o AR8171 PCIe Gigabit Ethernet controller
   o AR8172 PCIe Fast Ethernet controller
   o Killer E2200 Gigabit Ethernet controller

  Relnotes:	yes
MFC r272731:
  Document newly added controller AR816x/AR817x.
MFC r262294 (by ivoras):
Explain how and where kern.cam.ada.write_cache can be set in practical
situations.
MFC r262332 (by ivoras): Grammar fix
MFC r271926:  Mention read_ahead tunables/sysctls.
MFC r272683:
- Fix compile warning when compiling with GCC.
- Add missed chunk in previous driver code MFC.

Sponsored by:	Mellanox Technologies