Rollup merge of #155133 - cuviper:duration_fp_docs, r=the8472

Document precision considerations of `Duration`-float methods

A `Duration` is essentially a 94-bit value (64-bit sec and ~30-bit ns),
so there's some inherent loss when converting to floating-point for
`mul_f64` and `div_f64`. We could go to greater lengths to compute these
with more accuracy, like rust-lang/rust#150933 or rust-lang/rust#154107,
but it's not clear that it's worth the effort. The least we can do is
document that some rounding is to be expected, which this commit does
with simple examples that only multiply or divide by `1.0`.

This also changes the `f32` methods to just forward to `f64`, so we keep
more of that duration precision, as the range is otherwise much more
limited there.