Help: Unclear which project() calls CMAKE_PROJECT_INCLUDE may affect

In the original MR's discussion, it was noted that the file mentioned by CMAKE_PROJECT_INCLUDE may get included multiple times. The docs don't currently give the reader any guidance on this and they could reasonably (but incorrectly) guess that only the top level project is affected rather than all project() calls. The docs should be clarified to make clear that every project() call will include the named file.