Config Docs for [API]Client classes should include all values

[bshaffer]

Related to #615
The config parameter credentialsFetcher for StorageClient is supported, but not shown in the documentation.

This is because all config values cascade down to the RequestWrapper, so anything valid for the request wrapper will be valid for the clients. I propose adding all relevant config values to the [API]Client constructor docs as well for consistency. Most of them are already there, but requestTimeout and credentialsFetcher are missing.

[bshaffer]

Note: The credentialsFetcher parameter is supported in the GrpcRequestWrapper class as well via the RequestWrapper trait.

[bshaffer]

It appears the \Grpc\BaseStub and [API]ServiceGrpcClient classes do not utilize the RequestWrapper trait (yet), so these options are not available there.