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.

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