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
@@ -97,8 +97,9 @@ func (d *DocumentRef) Get(ctx context.Context) (*DocumentSnapshot, error) {
// - float32 and float64 convert to Double.
// - []byte converts to Bytes.
// - time.Time converts to Timestamp.
-// - latlng.LatLng converts to GeoPoint. latlng is the package
-// "google.golang.org/genproto/googleapis/type/latlng".
+// - *latlng.LatLng converts to GeoPoint. latlng is the package
+// "google.golang.org/genproto/googleapis/type/latlng". You should always use
+// a pointer to a LatLng.
// - Slices convert to Array.
// - Maps and structs convert to Map.
// - nils of any type convert to Null.
