It is almost impossible to remember the usage of the helper utility plugins in maven. Every single time I have problems with transient dependencies I end up searching google for "maven help plugin" and "maven dependency plugin". That is not good enough.
The help plugin should have a goal called help, describing the usage of the plugin. This would make help (or, rather, a bootstrap on how to use the help system) available under the obvious command "mvn help:help". This goal could also hint about the existence of the dependency plugin, since many of the difficult problems when using maven are related to complex transitive dependencies.
The command "mvn -help" should also describe the use of the maven-help-plugin, but I will create a separate issue in the maven core module for that.