docs: add return type explanation for score method

[gytic]

This PR improves the documentation for the pattern::source method by providing additional context for the return type.

The docstring now specifies, in which Cases None will be returned, as this is unintuitive otherwise.

If a rewording is wanted, please let me know.

[reneleonhardt]

@gytic Sorry, I have no maintainer permissions, you have to find someone else.