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_

[bernardnormier]

I believe the following local exceptions can be thrown by application code:

client-only mappings based on C++ (MATLAB, Ruby, PHP)

None. That's because these language mappings don't allow the application to register their own ValueFactoryManager.

C++, C#, Java, JavaScript

• AlreadyRegisteredException from a custom ValueFactoryManager (this ability is not documented for Python in Ice 3.7)
• the 6 special marshalable local exceptions (NotExistException and UnknownException)

Swift and Python

• the 6 special marshalable local exceptions (NotExistException and UnknownException)

Swift's InitializationData doesn't allow you to install a ValueFactoryManager. Python's InitializationData does but it's not used (i.e., it's a bug).