Help: Explain how target SOURCES are interpreted

Arcturusrequested to merge
arcturus140/cmake:22774-clarify-path-conversion-w.r.t-generator-expression into master

The merge request provides more detailed documentation and an example, which helps the reader to understand how relative paths are resolved for sources. This is a continuation of !6647 (closed).

Fixes: #22774 (closed)
Co-Authored-By: @craig.scott
Topic-rename: doc-SOURCES-genex

Edited by Brad King

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