15.0R/relnotes: add iwx(4), umb(4), umbctl(8)
Drivers iwx(4) and umb(4), and companion tool umbctl(8), are new for
15.0R.
Ref: src:2ad0f7e91582 (iwx)
Ref: src:0f1bf1c22a0c (umb)
Reviewed by: ziaee
Closes: https://github.com/freebsd/freebsd-doc/pull/564