mgb: Update man page wrt state of the driver

Be explicit that the driver has caveats and limitations, and remove the
note about not being connected to the build: I plan to connect it soon.
(Also the note serves no real purpose in a man page that is not
installed.)

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 667ea7385d9ce45c376260e9c2c893c51514e25e)