Clarify using BQ Storage client by default

Author: plamut

CHANGELOG.md

@@ -9,7 +9,7 @@
 
 ### Features
 
-* use BigQuery Storage client by default ([#55](https://www.github.com/googleapis/python-bigquery/issues/55)) ([e75ff82](https://www.github.com/googleapis/python-bigquery/commit/e75ff8297c65981545b097f75a17cf9e78ac6772)), closes [#91](https://www.github.com/googleapis/python-bigquery/issues/91)
+* use BigQuery Storage client by default (if dependencies available) ([#55](https://www.github.com/googleapis/python-bigquery/issues/55)) ([e75ff82](https://www.github.com/googleapis/python-bigquery/commit/e75ff8297c65981545b097f75a17cf9e78ac6772)), closes [#91](https://www.github.com/googleapis/python-bigquery/issues/91)
 * **bigquery:** add __eq__ method for class PartitionRange and RangePartitioning ([#162](https://www.github.com/googleapis/python-bigquery/issues/162)) ([0d2a88d](https://www.github.com/googleapis/python-bigquery/commit/0d2a88d8072154cfc9152afd6d26a60ddcdfbc73))
 * **bigquery:** expose date_as_object parameter to users ([#150](https://www.github.com/googleapis/python-bigquery/issues/150)) ([a2d5ce9](https://www.github.com/googleapis/python-bigquery/commit/a2d5ce9e97992318d7dc85c51c053cab74e25a11))
 * **bigquery:** expose date_as_object parameter to users ([#150](https://www.github.com/googleapis/python-bigquery/issues/150)) ([cbd831e](https://www.github.com/googleapis/python-bigquery/commit/cbd831e08024a67148723afd49e1db085e0a862c))