Add copy_sign() as doc alias for f32/f64::copysign()

[mqudsi]

This should help the compiler emit a more helpful error message when the
function is spelled the way it should have been.

cc #58046

[rust-highfive]

r? @kennytm

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[jyn514]

doc(alias) doesn't affect compiler error messages (although maybe it should? cc @estebank) - was that your goal? If so you should add a UI test, that would show that it has no effect.

[estebank]

We don't currently use doc(alias) for diagnostics, it is only used (afaik) to add alternative names in generated docs.

[mqudsi]

My mistake. Is there some other attribute that does help the compiler? (I think the compiler being aware of doc(alias) and using it for error messages is a good idea.)

[estebank]

@mqudsi we don't have such an attribute today, but I would love to see it happen at some point, whether it is by overloading doc(alias) or adding something else. I can help with it, but do not have the bandwidth at this time to take that project on myself.

[kennytm]

Currently searching copy_sign does give us {f32, f64}::copysign already. So I think we can close this PR.

[jyn514]

Closing this in favor of #83968.