Add Typeguard for Populated documents, or at least note that it is possible in the populate guide

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the issue has not already been raised

### Issue

This issue is a proposal to add a official `isPopulated(doc.path)` function to act as a typeguard both for types and in runtime.

A typeguard in typescript asserts that a input type is a specific type, and the same (or at least close to it) at runtime, for example (how typegoose does it):

```ts
function isPopulated(
doc: mongoose.PopulatedDoc
): doc is T extends mongoose.HydratedDocument ? mongoose.HydratedDocument : T extends object ? DocumentType : never {
return doc instanceof mongoose.Model;
}
```
Note: the typeguard above is a modified version from typegoose and the types have been changed to similar types in mongoose, but has not been tested, so it is just pseudo-code.

Also read [Typescript Narrowing: Using type predicates](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates).

Maybe a opposite function to check if the input is the ref type (ie unpopulated) would be a good idea; but from what i know it would not be just a simple invert of `isPopulated`.

If such a function is not added to mongoose itself, it should be noted in `docs/typescript/populate.html` that it is possible to outsource the check to a function while having the types still work.

Start by reading docs/typescript/populate.html and the TypeScript narrowing guidance linked in the issue. Decide whether the scope is a Mongoose runtime/typeguard API or documentation of an external check; done means the selected behavior is implemented and documented with compatible types and runtime behavior.

Written by the indexing model from the issue text.