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)

docs/man/docker-run.1.md

@@ -103,8 +103,10 @@ container can be started with the **--link**.
 **-m**, **-memory**=*memory-limit*
    Allows you to constrain the memory available to a container. If the host
 supports swap memory, then the -m memory setting can be larger than physical
-RAM. If a limit of 0 is specified, the container's memory is not limited. The 
-memory limit format: <number><optional unit>, where unit = b, k, m or g.
+RAM. If a limit of 0 is specified, the container's memory is not limited. The
+actual limit may be rounded up to a multiple of the operating system's page
+size, if it is not already. The memory limit should be formatted as follows:
+`<number><optional unit>`, where unit = b, k, m or g.
 
 **-P**, **-publish-all**=*true*|*false*
    When set to true publish all exposed ports to the host interfaces. The