Docs: cache policy callables get passed datastore API `Key`, not ndb `Key`

This took me a bit to debug just now, due to an error in the ndb docs, so I figured I'd file this to hopefully prevent that for others in the future. [The docs currently say](https://googleapis.dev/python/python-ndb/latest/context.html#google.cloud.ndb.context.Context.set_cache_policy) that the context `global_cache_timeout_policy` and (I think?) `global_cache_policy` callables get passed a `google.cloud.ndb.key.Key`, but that's not true. Evidently they actually get passed a [`google.cloud.datastore.key.Key`](https://cloud.google.com/python/docs/reference/datastore/latest/keys) instead.

Interestingly `cache_policy` gets passed a `google.cloud.ndb.key.Key`, which is documented correctly, but only adds to the confusion.

Hope this helps!