Update worker docs with recent enhancements (#7969)

changelog.d/7969.doc

docs/sample_config.yaml

docs/synctl_workers.md

docs/workers.md

synapse/app/generic_worker.py

synapse/config/federation.py

synapse/config/homeserver.py

synapse/config/logger.py

synapse/config/redis.py

synapse/config/workers.py

synapse/federation/send_queue.py

synapse/federation/sender/__init__.py

synapse/federation/sender/per_destination_queue.py

synapse/storage/data_stores/main/stream.py