Improve the "runtime is already installed" message

[pfmoore]

Is your feature request related to a problem? Please describe.
The warning that appears when you already have a particular version of Python installed and you do pymanager install is not very clear on the implications:

A runtime matching PythonCore\3.13 is already installed, and so the new one has not been registered

Describe the solution you'd like
It would be better if the message focused on the implications of not registering the new runtime, rather than on the technical detail of registration. Maybe something like

You already have an installation of PythonCore\3.13 - the new installation completed successfuly but some tools may still default to the old installation. When you remove the old installation, you will need to run py install --refresh to register this installation in its place.

(and can we not omit "PythonCore", on the basis that it's the default organisation key?)

Describe alternatives you've considered
Leave things as they are - the existing message isn't particularly bad, and it will generally only be seen by people managing multiple Python versions, who are more likely than average to be familiar with PEP 514 and the idea of registering installations in the Windows registry.