docs(python): align from_env secret messaging

[joshrotenberg]

Summary

• update the Python CloudClient.from_env() error message so it names every supported secret env var
• document the legacy key and secret aliases accepted by the Python binding
• strengthen the Python test to assert the alias names are present in the error message

Testing

• cargo test
• pytest python/tests/test_client.py (fails in local env: missing built redis_cloud.redis_cloud extension module during test collection)

Closes #58