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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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@ -3275,7 +3275,8 @@ Helper functions
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* Appends all values in `other_table` to `table` - uses `#table + 1` to
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find new indices.
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* `table.key_value_swap(t)`: returns a table with keys and values swapped
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* If multiple keys in `t` map to the same value, the result is undefined.
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* If multiple keys in `t` map to the same value, it is unspecified which
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value maps to that key.
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* `table.shuffle(table, [from], [to], [random_func])`:
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* Shuffles elements `from` to `to` in `table` in place
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* `from` defaults to `1`
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