Clarify key_value_swap's edge case (#10799)

In compiler design especially, leaving behavior as "undefined" is a _strong_ condition that basically states that all possible integrity is violated; it's the kind of thing that happens when, say, dereferencing a pointer with unknown provenance, and most typically leads to a crash, but can result in all sorts of spectacular errors--thus, "it is undefined" how your program will melt down.

The pure-Lua implementation of `key_value_swap` does not permit UB _per se_ (assuming the implementation of Lua itself is sound), but does deterministically choose the value to which a key is mapped (the last in visitation order wins--since visitation order is arbitrary, _some_ value _will_ be chosen). Most importantly, the program won't do something wildly unexpected.

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