Add PHPDoc for ServerStream iterator types

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:

```php
* @return ServerStream
```