docs: improve GitHub Apps compatibility info

[loozhengyuan]

With #753, users can use GitHub Apps as an authentication method but this is not compatible with all API endpoints. The provider documentation does include this disclaimer but does not indicate which data sources or resources are incompatible with it. If the auth method is incompatible with a particular data source or resource, it should be indicated accordingly so users can be better informed before they select a particular auth method or when they encounter API-related errors when using the GitHub Apps auth method.