Improve DeviceInitializationController Mock Parameter Handling in dodal

A brief description of the issue:
The mock parameter in the `DeviceInitializationController` API is only meaningful when `connect_immediately=True`. Otherwise, it is ignored, and users need to inspect the factory code to determine its behavior. This makes the API confusing and difficult to use, especially in cases requiring clarity on mock handling. There is a related [issue](https://github.com/DiamondLightSource/blueapi/issues/759) in blueapi to be worked on after this

This issue is not beamline-specific but arises from general usage challenges with the API. Addressing it will improve usability and make the API more intuitive for all developers.
Acceptance Criteria

- The behavior of the mock parameter is clearly documented, including its dependency on connect_immediately.
- Any misleading or unused functionality related to the mock parameter is addressed or renamed for clarity (e.g., mock_if_connect_immediately).
- Developers can easily understand how to configure mock without needing to inspect the factory code.