Add comment describing who can throw these exceptions

[pepone]

          It would be nice to add a comment describing who can throw these exceptions.

The NotExist exceptions and Unknown exceptions:

• application code
• Ice for Python code
• IcePy C++ extension code

All the other exceptions: only the IcePy C++ extension code?

Maybe users who implement a ValueFactoryManager are expected to throw AlreadyRegisteredException as well from their Python code? (seems undesirable - it's better if we can avoid having users throw local exceptions)

Originally posted by @bernardnormier in https://github.com/zeroc-ice/ice/pull/2417#discussion_r1667030816