Clarify validity requirements in docs of PackageFilesInfo/PackageFilegroupInfo

Is it invalid (meaning under no circumstances a good idea, assumed by consuming rules not to happen) to have
- A `PackageFilegroupInfo` that contains the same destination file twice? I assume yes, c.f. #682
- A `PackageFilesInfo` that contains directory `File`s?
- A `PackageFilesInfo` that contains symlink `File`s?
- One or two `PackageFilesInfo` in the same `PackageFilegroupInfo` containing both a file `a/b` and a file `a/b/c`, where `a/b` is a regular or symlink `File`?
- One or two `PackageFilesInfo` in the same `PackageFilegroupInfo` containing both a file `a/b` and a file `a/b/c`, where `a/b` is a directory `File`?

Imo it would be helpful to mention that in the docs of the respective provider. That way, these providers can become a more standardized interface between producing and consuming rules.