net: l2: ieee802154: document non-standard ACK feature selection

For IEEE 802.15.4 compliance, the NET_L2_IEEE802154_ACK_REPLY option must
automatically be active if the radio driver does not AUTOACK and inactive
otherwise. No user interaction is required.

Future changes will deprecate this option and replace it by a standard
compliant automatic mechanism.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>