Return-Types for Schema.prototype.discriminator do not align with runtime behavior

[hasezoey]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Currently Schema.prototype.discriminator takes the discriminator schema as a argument (argument number 2 of 2) and this is the parent schema the discriminator schema is applied on and returns this at the end.
But the types currently imply that this function returns the discriminator schema (the schema from the argument):

Runtime:

mongoose/lib/schema.js

Lines 528 to 532 in 4495cf3

	Schema.prototype.discriminator = function(name, schema) {
	this._applyDiscriminators = Object.assign(this._applyDiscriminators || {}, { [name]: schema });
	return this;
	};

Types:

mongoose/types/index.d.ts

Line 198 in 4495cf3

discriminator<T = Schema>(name: string, schema: T): DiscriminatorSchema<DocType, M, TInstanceMethods, TQueryHelpers, TVirtuals, T>;