Add documentation snippets for IN/NOT_IN/NOT_EQUAL operators

I work on the Firestore/Datastore documentation, and I'd like to add some example snippets for IN/NOT_IN/NOT_EQUAL operators.

File to update:
[samples/snippets/src/test/java/com/google/datastore/snippets/ConceptsTest.java](https://github.com/googleapis/java-datastore/blob/main/samples/snippets/src/test/java/com/google/datastore/snippets/ConceptsTest.java)

These snippets will depend on #688.
