Improve documentation of `vsmem_helper_default_fallback_policy_t`

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_

Locate the documentation for `vsmem_helper_default_fallback_policy_t` and compare its stated alias with `vsmem_helper_fallback_policy_t`. Correct the documentation so it names the actual aliased type. Done means the documentation no longer refers to `vsmem_helper_t`.

Written by the indexing model from the issue text.