Auto merge of #22076 - carols10cents:exp2-calling-exp, r=huonw

I was working on adding examples to the documentation in `std::num::Float`. I got to `exp2`, which says "Returns 2 raised to the power of the number, `2^(self)`."

So I tried running this code:

```
use std::num::Float;

#[test]
fn test_exp2() {
    assert_eq!(32.0, 5.0.exp2());
}
```

and it resulted in a failure of `(left: `32`, right: `148.413159`)`. That 148.413159 is the value for e^5, which is `exp()`, not `exp2()`.

Sure enough, `exp2` is calling `exp` and shouldn't be, looks like a copy-paste error. 

I haven't added any tests for this since it's unlikely to break again, but I will happily do so if people think that would be a good idea. The doc examples are coming :)

I scanned through the other functions in these files for similar sorts of errors and didn't notice any.

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