docs: terminals: mention subreaper requirement
docs: terminals: mention subreaper requirement

I realised that the terminal documentation which covers detached
terminals fails to mention that callers need to make themselves a
subreaper. Probably a good idea to mention this. I've also included a
minor comparison to LXC.

Signed-off-by: Aleksa Sarai <a8eb12d32d6ec07cdf737175ec7d4a0f1b4e1ee7@suse.de>

opencontainers/runc,opencontainers/runc,rhatdan/runc,mikebrow/runc,estesp/runc,opencontainers/runc,mikebrow/runc,dqminh/runc,dqminh/runc,dqminh/runc,estesp/runc,kolyshkin/runc,dqminh/runc,rhatdan/runc,xiaochenshen/runc,opencontainers/runc,estesp/runc,xiaochenshen/runc,rhatdan/runc,mikebrow/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,kolyshkin/runc,xiaochenshen/runc