Add php docblock for the logger option

Author: Hectorhammett

PubSub/src/PubSubClient.php

@@ -184,6 +184,9 @@ class PubSubClient
      *           See the {@see \Google\ApiCore\Transport\GrpcTransport::build()} and
      *           {@see \Google\ApiCore\Transport\RestTransport::build()} methods for the
      *           supported options.
+     *     @type false|LoggerInterface $logger
+     *           A PSR-3 compliant logger. If set to false, logging is disabled, ignoring the
+     *           'GOOGLE_SDK_PHP_LOGGING' environment flag
      * }
      * @throws \InvalidArgumentException
      */