Help: Add that CMAKE_CACHEFILE_DIR might not be defined

CMake only sets CMAKE_CACHEFILE_DIR when writing CMakeCache.txt, so the variable will usually be undefined when CMakeLists.txt runs. Revise its documentation to clarify that CMAKE_BINARY_DIR should be used instead.

I ran into this problem myself, and using CMAKE_BINARY_DIR works even before CMakeLists.txt has been written.

Backport: release