Add a comment showing mapping to radiotap.

Useful if we try to provide some "standard" 802.11 metadata header that
can support both radiotap and Peek tagged (and perhaps others).

Change-Id: Ibac9829e3411670a439db7cb77e1694a5641b0a5
Reviewed-on: https://code.wireshark.org/review/4970
Reviewed-by: Guy Harris <guy@alum.mit.edu>