Documentation: Improve Docstrings in modules google/pubsub_v1/services/subscriber

[gentris]

Improves docstrings on the following modules, by replacing returns with raises:

• google/pubsub_v1/services/subscriber/async_client.py
• google/pubsub_v1/services/subscriber/client.py
• google/pubsub_v1/services/subscriber/transports/grpc.py
• google/pubsub_v1/services/subscriber/transports/grpc_asyncio.py

Fixes #1020 🦕

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