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.
45 jobs for !7125 with help-variables-cmakecachefiledir-might-not-be-defined in 4 minutes and 8 seconds (queued for 4 seconds)
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