Add PHPDoc for ServerStream iterator types

[bshaffer]

We currently document ServerStream as the return type for server streaming types. This is not very useful, as we need to know what the type is that the ServerStream returns when you iterate over the results. Otherwise the only way for the customer to know what is returned is to make the call and wait, or to look at the source protos:

Example:

• Client call: https://github.com/googleapis/google-cloud-php/blob/bbe4abc29a9cf166d06649f6616ea0af609065f4/Spanner/src/V1/Client/SpannerClient.php#L592
• Source protos: https://github.com/googleapis/googleapis/blob/2d4fbff6d1f8e97ff077b49f8a4bc8ac663dc5d3/google/spanner/v1/spanner.proto#L151

A better solution would be to have the @return phpdoc look like this:

* @return ServerStream<PartialResultSet>