releases/15.0R/relnotes: Note wireless firmware on bootonly media

Wireless firmware packages are now included on the bootonly ISO and the
mini-memstick image so users can fetch installation files over a
wireless connection when using Wi-Fi drivers supported by these
firmware. Also make it clear, that the DVD ISO includes firmware for a
broader set of Wi-Fi drivers.

Reviewed by: bz, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53989