Rollup merge of #139099 - scottmcm:from_fn_docs, r=Amanieu

Promise `array::from_fn` is generated in order of increasing indices

Fixes #139061

I agree this needs to be documented because of the `FnMut`, either with a guarantee or to explicitly disclaim one.

I'm pretty sure this will be non-controversial (like the other "well sure you *could* do it in a different order, but why?" things were), but I couldn't find any previous libs-api decision on it so it's seemingly a new promise that will need FCP.

Basically, yes, it would be plausible to fill in the reverse order, but there's no obvious way we could ever know that that might even be a good idea, so forward seems like an easy thing to promise.  We could always add a `from_fn_rev` or something later if there's ever a strong enough need, but it seems unlikely.

Let's just do the obvious thing so it matches what `[gen(0), gen(1), …, gen(N-1)]` does.

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