Add guard to stop loadClass copying Document if Document is used as base of loaded class (same hack as implemented for Model already).

[sgpinkus]

Summary
I want to set Document as the base class of a loadClass class so I can inherit its typings on this conveniently. There is a guard condition hack in the code to allow this for Model already. This PR just extends to guard condition to account for possibility Document is the base class. If that didn't already exist I wouldn't PR this, but since it exists may as well account for Document too, in case people do that, and for consistency.

Examples
Something like this:

export class MyResourceMethods extends Document
{
  delete() {
    return this.remove();
  }
  async ping() {
    await this.updateOne({ $inc: { pingCount: 1 } });
    return { modified: true };
  }
 ...
}

If you want Typescript to know types for Document methods (updateOne, remove in example) you can either type this in every method or do the above. The above is more convenient.

Refs #12813.

[vkarpov15]

Thanks 👍