MatRangeDateSelectionModel should be excluded from public API

[MursalovAltun]

Documentation Feedback

Any usage example of injectable service MatRangeDateSelectionModel

Affected documentation page: https://material.angular.io/components/datepicker/examples

[jelbourn]

@crisbeto is this class mostly used internally?

[crisbeto]

Yes, the model is used internally to sync up the value between the various datepicker components. We should mark it as @docs-private so that it doesn't show up in the docs.