`<{f16,f32,f64,f128} as Rem>::rem` documented definition is misleading w.r.t. intermediate rounding

Our documentation for impl Rem for {f16, f32, f64, f128} says:

The remainder has the same sign as the dividend and is computed as: x - (x / y).trunc() * y.

But that's not true. E.g.:

fn main() {
    let (x, y) = (11f64, 1.1f64);
    assert_eq!(x - (x / y).trunc() * y, x % y);
    //~^ PANIC
    // assertion `left == right` failed
    // left: 0.0
    // right: 1.0999999999999992
}

This mismatch creates a hazard when trying to correctly encode algorithms that rely on this semantic.

This tripped me up, e.g., when authoring:

• https://github.com/rust-lang/rust/pull/133755

cc #133485 #107904

cc @cuviper @Noratrieb @tczajka @Neutron3529 @BartMassey

Start with the linked Rem implementation documentation and the displayed formula, then reproduce the Rust example with f64 values to observe the intermediate-rounding mismatch. Revise the documentation so it does not describe % as exactly that real-number formula, and verify that the updated wording accurately matches the demonstrated behavior.