Help: Document warning about multiple source paths

Brad Kingrequested to merge
brad.king/cmake:doc-multiple-src-paths into master

Although passing multiple source paths was never documented, it was not diagnosed by CMake 3.22 and below. In CMake 3.23 we now diagnose extra paths and warn. Document this change and add a release note.

Issue: #23334 (closed)
Backport: release

Edited by Brad King

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