Document powf and powi values that are always 1.0

f64 part for bug #90429

Author: hkBst

library/std/src/f64.rs

@@ -306,8 +306,9 @@ impl f64 {
     /// ```
     /// let x = 2.0_f64;
     /// let abs_difference = (x.powi(2) - (x * x)).abs();
-    ///
     /// assert!(abs_difference < 1e-10);
+    ///
+    /// assert_eq!(f64::powi(f64::NAN, 0), 1.0);
     /// ```
     #[rustc_allow_incoherent_impl]
     #[must_use = "method returns a new number and does not mutate the original value"]
@@ -329,8 +330,10 @@ impl f64 {
     /// ```
     /// let x = 2.0_f64;
     /// let abs_difference = (x.powf(2.0) - (x * x)).abs();
-    ///
     /// assert!(abs_difference < 1e-10);
+    ///
+    /// assert_eq!(f64::powf(1.0, f64::NAN), 1.0)
+    /// assert_eq!(f64::powf(f64::NAN, 0.0), 1.0)
     /// ```
     #[rustc_allow_incoherent_impl]
     #[must_use = "method returns a new number and does not mutate the original value"]