Better usability for creating Port bindings

I found that the syntax for creating port bindings is hard to memorize, especially if you also use the docker CLI (see [issue comment](https://issues.jenkins-ci.org/browse/JENKINS-23704?focusedCommentId=207190&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-207190)).

This PR documents legal syntax in a help message, input validation and tests.
The actual parsing algorithm was not touched.