Document collection From and FromIterator impls that drop duplicate keys.

[kpreid]

This behavior is worth documenting because there are other plausible alternatives, such as panicking when a duplicate is encountered, and it reminds the programmer to consider whether they should, for example, coalesce duplicate keys first.

Followup to #89869.

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[the8472]

You're documenting the order here, this constrains future implementation changes.

And you're sometimes saying values and sometimes entries. That'd be even more constrianing because it means that either only values or keys and values must be replaced, depending on the method used, which can make a difference for keys that are equal but not identical.

[kpreid]

Good point — I wasn't thinking of an appropriate middle ground between “no comment” and “document the precise behavior”. I have now updated the text so that in all cases it says

all but one (of the corresponding values)? will be dropped

which should be broad enough to permit all possible treatments of ordering.