Document tearing consideration

[madelson]

Because we're a struct without atomic writes, you can corrupt ILL instances by overwriting them on multiple threads.

For the same reason, we can't support Interlocked assignments.

One option would be to make ILL a class instead of a struct, although that means 2 allocations per update instead of just one. Another option would be to carry count in every node which increases space. This could be ameliorated by using "fat" nodes (multiple values per node), but that would have other costs.