Making stubs for auto-generated equivalence methods for documentation.

[wwitzel]

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Currently, the verb and past-tense forms of equivalence methods are auto-generated. For example, you register "simplification" as an equivalence method and then "simplified" and "simplify" are automatically generated. Documentation is also auto-generated, but it won't show up in the Sphinx documentation pages and doesn't show up in the list of methods in the class declaration. It would probably be best to put the method stubs in there show they show up. It is a little trickier though in the "simplify" case and things like it, since the method is only applicable when the expression is wrapped in the InnerExpr class. But we could still have the stub there with documentation explaining that. Maybe there is another solution, but this would be better than what currently exists where these auto-generated methods are obscure.