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

[SpaceIm]

Indeed, I've tested during migration of a recipe to conan v2 to pass a CMAKE_POLICY_DEFAULT_CMP<NNNN> 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.