Document memory limit sizing in manpages

The -m flag permits the setting of a memory limit when running a Docker
container. The actual limit set must be a multiple of page size on Linux, so
whatever number the uses passes in will be rounded up if needed. Document this
behavior to prevent confusion. Also fixed several small formatting and grammar
issues in the docker run manpage.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)
Merge pull request #6680 from mheon/cgroup_memory

Update docker-run manpage