Help: Clarify when CMAKE_VERIFY_INTERFACE_HEADER_SETS can be modified by the project

The previous wording claimed projects should not set this variable, that only the developer should do so. But that did not consider code brought in from external dependencies, which the developer would not normally want to verify. Modify the wording to clarify this particular scenario, and provide a clear example for how to follow the recommended arrangement in such cases.

Fixes: #23808 (closed)
Backport: release