message: docs(virtuals): improved description of ref option
diff: @@ -58,7 +58,7 @@ Virtual-Populate is also supported by Typegoose
  Options ([look here for more details](https://mongoosejs.com/docs/api/schema.html#schema_Schema-virtual)):
  -- `ref`: This is like a normal ref **[Required]**
  +- `ref`: This is like a normal [ref](https://typegoose.github.io/typegoose/docs/api/decorators/prop/#ref), use `'ClassName'` when the classes are in different files **[Required]**
  - `foreignField`: Which property(on the ref-Class) to match `localField` against **[Required]**
  - `localField`: Which property(on the current-Class) to match `foreignField` against **[Required]**
  - `justOne`: Return as One Document(true) or as Array(false) ***[Optional]***
  