message: docs(ref): add different "_id" type example
diff: @@ -13,6 +13,8 @@ For an more written out guide, there is the [Reference Other Classes](../../guid
  ## Example
  +Referenced Class in the examples:
  +
  ```ts
  class Kitten {
  @prop()
  @@ -20,6 +22,8 @@ class Kitten {
  }
  ```
  +Reference Array:
  +
  ```ts
  class Cat {
  @prop()
  @@ -31,6 +35,8 @@ class Cat {
  }
  ```
  +Single Reference:
  +
  ```ts
  class Person {
  @prop()
  @@ -41,3 +47,22 @@ class Person {
  public pet?: Ref<Cat>;
  }
  ```
  +
  +Reference with different `_id` type:
  +
  +```ts
  +class Cat {
  + @prop()
  + public _id: string;
  +}
  +
  +class Person {
  + @prop()
  + public name: string;
  +
  + // The "type" (_id type) needs to be manually set, otherwise mongoose will default to "ObjectId"
  + // which is not compatible with other types (ObjectId !== String)
  + @prop({ ref: () => Cat, type: () => String })
  + public pet?: Ref<Cat, string>;
  +}
  +```
  