[MNG-7937] Add description of mojo complex object types

**[Ben Tatham](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=bentatham@nanometrics.ca)** opened **[MNG-7937](https://issues.apache.org/jira/browse/MNG-7937?redirect=false)** and commented

Currently, when a mojo uses "complex types" as parameter types, trying to discover what fields are available in those types,. and that those fields do, is difficult. We rely on external documentation or source code itself.

This would be helpful for custom plugins, but also for the common public plugins as well (what are the fields of `Artifact` again?)

It would be great if the maven-plugin-plugin generated information about these complex types, hopefully pulling javadoc and/or new annotation information from the type fields themselves. 

I think this would require updates to a few places:
* maven-plugin-plugin to generate the information
* the plugin xml itself, to describe these types.
* maven-help-plugin to display this information.
* plugin report generation to include the type information in the plugin/mojo documentation.
* (and then tools/IDE's etc could add features to support this as well with auto-complete, etc)

Allowing `@Parameter` (or some new field annotation) on the fields to allow aliases, etc would also be helpful, but perhaps not required/separate feature.

An example of documentation that could easily be auto-generated and available via help:describe with this feature: [RuleSet](https://www.mojohaus.org/versions/versions-maven-plugin/version-rules.html#using-the-ruleset-element-in-the-pom).

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No further details from [MNG-7937](https://issues.apache.org/jira/browse/MNG-7937?redirect=false)