Document `torch.utils.cmake_prefix_path` (#38727)
Summary:
Documents new global variable pointing to PyTorch CMake config files
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38727
Differential Revision: D21694243
Pulled By: malfet
fbshipit-source-id: 652532cd5da9945caf7d7dfe1fde696dc474661b
