Merge topic 'doc-SOURCES-genex'

9fac18a4a6 Help: Clarify target_sources path conversion w.r.t generator expressions
9abd63dd3a Help: Explain how target SOURCES are interpreted

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Michael Hirsch <michael@scivision.dev>
Merge-request: !6692