add $apiKey to documented constructor options

The [ClientOptions](https://docs.cloud.google.com/php/docs/reference/gax/latest/Options.ClientOptions#_Google_ApiCore_Options_ClientOptions__setApiKey__) class supports `apiKey`, but it's not documented in the client constructor. This is probably because it isn't supported everywhere, but we can add to the documentation that it might not be supported. The API throws a "Not Implemented" error if it isn't supported, so there's no reason to not document it.