I'm not looking forward to documenting cli arguments that may or may not show depending on what plugins / drviers you choose

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #4786 from SvenDowideit/run--lxc-conf-is-lxc-driver-only-atm

add a note that --lxc-conf is lxc-driver only.