firestore: fit & finish for `OrFilter`, `AndFilter`, `PropertyFilter`

The [implementation of the `OrFilter`](https://github.com/googleapis/google-cloud-go/commit/ae377932de20d99f31766ca1cccd2d1cfa18a1c0) introduces a new concept into the Firestore client library: `PropertyFilter`.

However, this `PropertyFilter` has a couple of deviations from the typical design of the Firestore client:

+ It duplicates the features of queries. The `WhereEntity()` method with a single property filter is very similar to the `Where()` method with arguments supplied.
+ It refers to [Document fields](https://cloud.google.com/firestore/docs/data-model) as "Properties" or "Paths", which is a new concept.
+ It doesn't use the [fluent API](https://en.wikipedia.org/wiki/Fluent_interface) design used elsewhere in the client library.

Additional context from snippets usage can be found [here](https://github.com/GoogleCloudPlatform/golang-samples/pull/2989).