Help: Mention in find_package that cmake-gui step is Windows-only (#14781)

Recently used cmake-gui locations are searched only on Windows because
the Windows registry is used to record the values.  This behavior is
historical and may be removed by a policy in the future so rather than
implementing it on other platforms simply document the current behavior.
Merge topic 'doc-find_package-build-paths' into next

69d73d27 Help: Mention in find_package that cmake-gui step is Windows-only (#14781)