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 a formatting issue in the docker run
manpage.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)