[MPLUGIN-536] Allow generation of documentation for nested POJOs used as @Parameters

**[Ashley Scopes](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=JIRAUSER291213)** opened **[MPLUGIN-536](https://issues.apache.org/jira/browse/MPLUGIN-536?redirect=false)** and commented

As a plugin developer of a plugin with a few dozen complex options, I'd like to see a new feature where by the generated site documentation and help mojo document the fields of nested pojos that are present with javadoc comments.

```java
@Parameter
private Thing thing;
```

```java
public class Thing {
public Foo getFoo() { ... }
public void setFoo(Foo foo) { ... }
}
```

Currently the plugins just report the fully qualified class name as a dead link, which means nested attributes have to be documented manually and the consistency of this varies wildly between projects.

I'd either like to see the ability for the documentation to be generated from project sources, dependency sources, or JavaDoc JARs, or the ability to provide a custom `@Documentation` annotation to document attributes in nested objects.

This could be specified within a "schemas" section at the bottom of the goal HTML page.

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No further details from [MPLUGIN-536](https://issues.apache.org/jira/browse/MPLUGIN-536?redirect=false)