ure.4: description consistencies + tag spdx

+ consistent document description languague with other USB-BaseT drivers
+ mention newly added adapters from 6ea4d9
+ attempt to mention rgephy(4) phys feed into ure interfaces

Fixes: 6ea4d9 (Move RTL8156 from cdce(4) to ure(4))
MFC after: 3 days

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1384

(cherry picked from commit 4c72525953fdf618a3fc0a45f3ef3f071dcc0c52)