Add Doxygen markup for `@unstable` or similar

[foundrytom]

What

Ensure it is clear in the API documentation which methods are considered unstable (ie: yet to be migrated/updated).

Why

As we build more PoCs around the alpha builds, it is important for developers to know where they should expect churn, and which parts are "stable".

ACs

• A Doxygen alias: @unstable that expands to @warning This method or class has not been updated for the new C++ API and may be subject to a changes in signature in subsequent releases.
• Add these to all methods in ManagerInterface.py/Manager.py (ie. that haven't been ported to C++).
• Classes in terminology.py and exceptions.py