Clarify requirement for format of URI-valued properties for nested files

[ghost]

In the section on run.files we specify the format of the property names in the files object, including the case where the property describes a nested file. In that case, we explain how to use the URI fragment to specify the location of the nested file with respect to its parent.

But nowhere do we state that when a URI-valued property specifies a location in a nested file, the value of that property should be specified in that same way.