groovydoc: some __$stMC field is inherited from base class

Not sure if this is a bug or an improvement suggestion: hide __$stMC inherited field.

Steps to reproduce

Given a base class:

package base

class Base {
}

In the real world this is provided by another project.

The base class can also be abstract, but is has to be a Groovy class.

And compile the base class, because it needs to be on the classpath when documenting the derived class. Otherwise this bug won't show up.

groovyc -d=classes Base.groovy

And, have a derived class:

import base.Base

class Derived extends Base {
}

When API docs are generated for this derived class:

groovydoc -classpath classes --destdir=docs Derived.groovy

Then an inherited field __$stMC is shown in the docs:

Technically those docs are probably telling the truth, but I think it should be hidden.