Rollup merge of #47277 - tspiteri:log-correctness, r=frewsxcv

Rollup merge of #47277 - tspiteri:log-correctness, r=frewsxcv

doc: show that `f32::log` and `f64::log` are not correctly rounded

Fixes #47273.

One thing I'm not sure about is whether the "calculated as `self.ln() / base.ln()`" bit is being too specific, maybe we do not want to make this such a strong commitment. I think it's fine, but we should not make commitments in the API documentation by accident.

In case that is removed, the added sentence "`self.log2()` can ... base 10." still makes it amply clear that the `log` methods can be more inaccurate than other methods. If the above clause is removed, this second sentence can be moved to the first paragraph, kind of like the accuracy comment for the [`mul_add`](https://doc.rust-lang.org/std/primitive.f32.html#method.mul_add) method.