net80211: document the crypto enmic/demic functions.

These functions implement what's needed for TKIP Michael MIC - which
is performed over the entire unencrypted MSDU. Each potential
fragmented MPDU is encrypted and has its own ICV/MIC.

CCMP/GCMP encrypts each MPDU separately (including the MPDUs that make
up an A-MPDU), so they'll implement null functions here and instead do
the MIC/ICV insertion inline in the crypto functions themselves.

Hopefully this makes it a bit clearer on how things should behave, and
will help figure out and clean up what further hardware offload
features we need.

Differential Revision: https://reviews.freebsd.org/D49392
Reviewed by: bz