firestore: document that a pointer to a LatLng should be used.
firestore: document that a pointer to a LatLng should be used.

Clarify that the right type for GeoPoint is *latlng.LatLng, not latlng.LatLng.

Fixes #962.

Change-Id: Iaead92e547e57230c1f31b6726a12eeda3535c1f
Reviewed-on: https://code-review.googlesource.com/26470
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go