Help: Reorganise FILE_SETS and related properties

Reduce duplication and make the docs for target_sources() focus on the functionality rather than the properties it modifies. The properties are a lower level quantity, so put the relevant details for them in the property documentation. The target_sources() command only needs to reference the properties, not reproduce the property documentation.

Improve the cross-referencing between the HEADER_... property docs. This helps build the mental picture of how they relate to each other.

Backport: release