Improve documentation of `vsmem_helper_default_fallback_policy_t`

[miscco]

          The documentation is lying to me. It says the alias is to `vsmem_helper_t`, but it aliases to `vsmem_helper_fallback_policy_t`. This may be a copy&paste error from above?

Originally posted by @bernhardmgruber in https://github.com/NVIDIA/cccl/pull/1643#discussion_r1589338190