NODE-4007 Description

How are you using Mongo? What version of the server and driver are you using?

I'm using the npm mongodb version 4.2.2.

What is the feauture/improvement you would like?

The typing declares `interface FilterOperators<TValue> extends Document { ... }`

I think you mean `interface FilterOperators<TValue extends Document > { ... }`

What use case would this feature/improvement enable?

Currently, `findOne<TSchema> (filter: Filter)` can accept anything as it's arguement (not much type safety).  This is because `Filter` extends `RootFilterOperators` which extends `Document`:

```
Filter<TSchema> =

& RootFilterOperators<WithId<TSchema>>;
```

I think you mean for `RootFilterOperators` to only be a bunch of keys that start with "$" (e.g. `$eq`, `$gt`, etc ...).