ScratchMemorySpace not documented

[nmm0]

Also related: #536

In general I think our hierarchical parallelism documentation needs to be improved.

[masterleinad]

I'm not sure we actually want to expose the ScratchMemorySpace API to users. All they need from it is the type to be used in an unmanaged View. Everything else are implementation details.

[masterleinad]

kokkos/kokkos#2233 discusses if ScratchMemorySpace should be a proper memory space.

[nmm0]

I'm not sure we actually want to expose the ScratchMemorySpace API to users. All they need from it is the type to be used in an unmanaged View. Everything else are implementation details.

That's fair though unfortunately it is referenced in our documentation by other things. So even if we don't want a full-blown API reference we at least need a page to explain what it is.

[masterleinad]

Note that we are still debating how much of the memory space API we really want users to use.