CheckIPOSupported: Document support for CUDA

Support was added for CMake 3.25 by commit 96bc59b1ca (CUDA: Add Device
LTO support for nvcc, 2022-04-22, v3.25.0-rc1~359^2).  While at it:

- Adjust the note about the required policy.
- Document which languages are checked when the LANGUAGES
  option is not specified.
1 file changed