Clarify assembly review

https://github.com/dalek-cryptography/subtle/blob/6b6a81ad9a6a00c0b42c327eaf4b2f785774377e/src/lib.rs#L332-L338 Doesn't really rule out a simple `if x == 0 goto end` optimization. I assume the assembly was inspected but that should probably be annotated because it would have to be repeated when the compiler does smarter things.