net80211: document the vap / device transmit paths, fragment node references

• Document what the VAP and device transmit paths do. Take care to note down the 802.11 fragment handling and node references.

• Almost none of the drivers (save ath(4), which I fixed a long time ago) implement 802.11 fragments after the big VAP rework done years and years ago. A few others announce support (mwl, ral, uath, wpi) but I haven't validated them. I'll be doing that as part of this work.

• Add some comments in ieee80211_fragment() around how fragment node references work.

Differential Revision: https://reviews.freebsd.org/D49765