[`Snapshot.execute_sql`](https://github.com/googleapis/google-cloud-python/blob/5756bf3b8f6cc935849f928383959386def17dd3/spanner/google/cloud/spanner_v1/snapshot.py#L154) is missing from the [API docs for `Snapshot`](https://googleapis.github.io/google-cloud-python/latest/spanner/snapshot-api.html). The public docs [tell developers to use this method](https://cloud.google.com/spanner/docs/getting-started/python/#using_the_client_library_for), so I suspect it should be in the API docs.