Expose tests for {f32,f64}.total_cmp in docs, along with comment

Uncomment the assert! line and account for the sign of NaN not
being positive, necessarily.

library/core/src/num/f32.rs

@@ -1424,9 +1424,16 @@ impl f32 {
     /// ];
     ///
     /// bois.sort_by(|a, b| a.weight.total_cmp(&b.weight));
-    /// # assert!(bois.into_iter().map(|b| b.weight)
-    /// #     .zip([-5.0, 0.1, 10.0, 99.0, f32::INFINITY, f32::NAN].iter())
-    /// #     .all(|(a, b)| a.to_bits() == b.to_bits()))
+    ///
+    /// if f32::NAN.is_sign_negative() {
+    ///     assert!(bois.into_iter().map(|b| b.weight)
+    ///         .zip([f32::NAN, -5.0, 0.1, 10.0, 99.0, f32::INFINITY].iter())
+    ///         .all(|(a, b)| a.to_bits() == b.to_bits()))
+    /// } else {
+    ///     assert!(bois.into_iter().map(|b| b.weight)
+    ///         .zip([-5.0, 0.1, 10.0, 99.0, f32::INFINITY, f32::NAN].iter())
+    ///         .all(|(a, b)| a.to_bits() == b.to_bits()))
+    /// }
     /// ```
     #[stable(feature = "total_cmp", since = "1.62.0")]
     #[must_use]

library/core/src/num/f64.rs

@@ -1422,9 +1422,19 @@ impl f64 {
     /// ];
     ///
     /// bois.sort_by(|a, b| a.weight.total_cmp(&b.weight));
-    /// # assert!(bois.into_iter().map(|b| b.weight)
-    /// #     .zip([-5.0, 0.1, 10.0, 99.0, f64::INFINITY, f64::NAN].iter())
-    /// #     .all(|(a, b)| a.to_bits() == b.to_bits()))
+    /// ```
+    /// Since the NaN constant does not have a well-defined sign,
+    /// this test may result in either:
+    /// ```
+    /// assert!(bois.into_iter().map(|b| b.weight)
+    ///     .zip([-5.0, 0.1, 10.0, 99.0, f64::INFINITY, f64::NAN].iter())
+    ///     .all(|(a, b)| a.to_bits() == b.to_bits()))
+    /// ```
+    /// or:
+    /// ```
+    /// assert!(bois.into_iter().map(|b| b.weight)
+    ///     .zip([f64::NAN, -5.0, 0.1, 10.0, 99.0, f64::INFINITY].iter())
+    ///     .all(|(a, b)| a.to_bits() == b.to_bits()))
     /// ```
     #[stable(feature = "total_cmp", since = "1.62.0")]
     #[must_use]