Added documentation for special case in run command

At present, starting a Docker container with a memory limit of 0 (docker run -m 0) does not apply a memory limit. This could be fixed, but it's fairly harmless behaviour and may even be desirable in some cases (make it explicit that a container can use as much memory as is available). This documents the behaviour in the man pages, to alleviate any confusion that could arise from this special case.