Try to explain that `ENTRYPOINT "" is immutable

Try to explain that `ENTRYPOINT ls -la /etc` is immutable, whereas
`ENTRYPOINT [\"ls\"] CMD [\"-la\", \"/etc\"] allows the parameters to
`ls` to be modified using  `docker run image /bin`

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)

Closes #3751
\`-d\`