fix(types): remove discriminator type requirement

[DouglasGabr]

Summary

As described in the issue #10421, the .discriminator function is yielding an error when trying to use an interface that doesn't extends Document.
Since Model enforces Document when needed, the discriminator type parameter doesn't need to extend it.

Fixes #10421

Examples

import { Model, model, Schema } from 'mongoose';

const options = { discriminatorKey: 'kind' };
interface IEvent {
  time: Date;
  kind: string;
}

const eventSchema = new Schema<IEvent, Model<IEvent>>({ time: Date }, options);
const Event = model<IEvent, Model<IEvent>>('Event', eventSchema);

interface IClickedLinkEvent extends IEvent {
  url: string;
}

// ClickedLinkEvent is a special type of Event that has
// a URL.
const ClickedLinkEvent = Event.discriminator<
  IClickedLinkEvent, // <-- no more error here
  Model<IClickedLinkEvent>
>('ClickedLink', new Schema({ url: String }, options));

[vkarpov15]

Thanks 👍