help:describe should be way more helpful when no arguments are provided

Right now if you "mvn help:describe" with no further arguments you get an error: "You must either specify 'groupId' and 'artifactId', or a valid 'plugin' parameter."

The people who are using help:describe don't necessarily know how help:describe works yet; if they did, they might be clever enough to use "mvn help:describe -Dplugin=help -Dmojo=describe" to figure out how help:describe works.

This error message should be more like a paragraph long, and should include valid working examples. It could be copied from the apt doc for the help plugin.