This fix made several updates:
1. Update opts.MemBytes so that default value will not show up.
The reason is that in case a default value is decided by daemon,
instead of client, we actually want to not show default value.
2. Move `docker run/create/build` to use opts.MemBytes for `--shm-size`
This is to bring consistency between daemon and docker run
3. docs updates.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>