Documentation inconclusive for `result_of` and `<cuda/std/type_traits>`

I searched the libcudacxx docs for `result_of` but didn't get any results.

After some time, I was able to find this page: https://nvidia.github.io/libcudacxx/standard_api/utility_library.html

However, `` appears to be missing from the sidebar.

In my understanding, `std::result_of` won't correctly check device lambda return types in host code, but `cuda::std::result_of` will check this and require `cuda::proclaim_return_type` (#284). This is the kind of difference that I would like to see documented.

It seems like the libcudacxx docs might expect users to look at both libcudacxx docs _and_ cppreference to figure out what headers contain a given functionality. I would prefer that libcudacxx docs are self-contained and link out for supporting information but not for discovering the contents of a given header. I'm not quite sure what libcudacxx docs are intended to provide, so I'm not sure what PR I could file to improve the situation.