CMakeToolchain: document that `variables` set variables with CACHE keyword

Indeed, I've tested during migration of a recipe to conan v2 to pass a `CMAKE_POLICY_DEFAULT_CMP` with `variables` and it was just ignored by CMake. I had to use `cache_variables`.

So it would be nice to document this behavior, and maybe provide some examples of the kind of variables which should be passed with `variables` or `cache_variables`.