Rollup merge of rust-lang#104074 - yancyribbens:add-example-to-round, r=Mark-Simulacrum

rustdoc: Add an example for round that is different from truncate

The current examples for [round](https://github.com/rust-lang/rust/blob/master/library/std/src/f64.rs#L75) would have the same results as the example for [truncate](https://github.com/rust-lang/rust/blob/master/library/std/src/f64.rs#L95).  This PR adds one more example to `round` that will have a different result from `truncate`.

Author: Yuki Okushi

Diff for: library/std/src/f32.rs

@@ -77,9 +77,11 @@ impl f32 {
     /// ```
     /// let f = 3.3_f32;
     /// let g = -3.3_f32;
+    /// let h = -3.7_f32;
     ///
     /// assert_eq!(f.round(), 3.0);
     /// assert_eq!(g.round(), -3.0);
+    /// assert_eq!(h.round(), -4.0);
     /// ```
     #[rustc_allow_incoherent_impl]
     #[must_use = "method returns a new number and does not mutate the original value"]

Diff for: library/std/src/f64.rs

@@ -77,9 +77,11 @@ impl f64 {
     /// ```
     /// let f = 3.3_f64;
     /// let g = -3.3_f64;
+    /// let h = -3.7_f64;
     ///
     /// assert_eq!(f.round(), 3.0);
     /// assert_eq!(g.round(), -3.0);
+    /// assert_eq!(h.round(), -4.0);
     /// ```
     #[rustc_allow_incoherent_impl]
     #[must_use = "method returns a new number and does not mutate the original value"]