docs: add brief note on TypeScript generic usage for embedded discriminator path() calls

[vkarpov15]

Re: #10435

Summary

Added back embedded discriminator docs, with a couple of comments on path() generic requirement for TypeScript.

I'll take another look and see if we can maybe remove the need to do schema.path<Schema.Types.Subdocument> and just have Mongoose infer the type correctly.

Examples

[vkarpov15]

No luck on getting TypeScript to automatically infer the schematype from path() calls. The closest I got was the following:

type SchemaTypeFromType<T> = T extends Types.DocumentArray<any> ? Schema.Types.DocumentArray : SchemaType<T>;

    path<pathGeneric extends keyof IfAny<EnforcedDocType, {}>>(path: pathGeneric): SchemaType<IfAny<EnforcedDocType, {}>[pathGeneric]>;
    path<pathGeneric extends keyof IfAny<DocType, {}>>(path: pathGeneric): SchemaTypeFromType<IfAny<DocType, {}>[pathGeneric]>;

However, with those changes, I get a bunch of test failures like the following:

  ✖  130:2  Type Schema<IProfile, Model<IProfile, {}, {}, {}, Document<unknown, {}, IProfile> & IProfile & { _id: ObjectId; }, any>, ... 6 more ..., Document<...> & ... 1 more ... & { ...; }> is not assignable to type SchemaDefinitionProperty<IProfile> | undefined.
  Type Schema<IProfile, Model<IProfile, {}, {}, {}, Document<unknown, {}, IProfile> & IProfile & { _id: ObjectId; }, any>, ... 6 more ..., Document<...> & ... 1 more ... & { ...; }> is not assignable to type Schema<any, any, any, {}, {}, {}, DefaultSchemaOptions, { [x: string]: any; }, Document<unknown, {}, { [x: string]: any; }> & { [x: string]: any; } & Required<{ _id: unknown; }>>.
    Types of property obj are incompatible.
      Type { age?: SchemaDefinitionProperty<number> | undefined; } is not assignable to type { [path: string]: SchemaDefinitionProperty<undefined>; } | { [x: string]: SchemaDefinitionProperty<any> | undefined; }.
        Type { age?: SchemaDefinitionProperty<number> | undefined; } is not assignable to type { [x: string]: SchemaDefinitionProperty<any> | undefined; }.
          Property age is incompatible with index signature.
            Type SchemaDefinitionProperty<number> is not assignable to type SchemaDefinitionProperty<any> | undefined. 

Seems like accessing keyof DocType in a generic throws TypeScript off. I'm just going to merge this PR, and take a look at getting TypeScript to correctly infer schema types when calling path() another time.