Fix atan2 inaccuracy in documentation

[DimitriiTrater]

Fixes #136275

[rustbot]

r? @tgross35

rustbot has assigned @tgross35.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[GrigorenkoPV]

Maybe it would make more sense to rephrase the existing part in terms of >= +0.0 and <= -0.0 instead of adding a new section?

View changes since this review

[DimitriiTrater]

If you take into account the signs of zeros in the function definition, then it will not make mathematical sense.
Signs of zeros in this context make sense only if we are talking about IEEE-754.
If you do it the way you say, then I need to remove the mathematical definition of the function from arctan and leave only the areas of incoming and outgoing values.

[DimitriiTrater]

I can suggest this:
x >= +0, y >= +0: [0, π/2]
x <= -0, y >= +0: [π/2, π]
x >= +0, y <= -0: [-π/2, 0]
x <= -0, y <= -0: [-π, -π/2]

@GrigorenkoPV what do you think?