Document powf and powi values that are always 1.0 (f16)

f16 part for bug rust-lang#90429

Author: hkBst

library/std/src/f16.rs

@@ -324,6 +324,15 @@ impl f16 {
     ///
     /// The precision of this function is non-deterministic. This means it varies by platform,
     /// Rust version, and can even differ within the same execution from one invocation to the next.
+    /// # Examples
+    ///
+    /// ```
+    /// let x = 2.0_f16;
+    /// let abs_difference = (x.powi(2) - (x * x)).abs();
+    /// assert!(abs_difference < f16::EPSILON);
+    ///
+    /// assert_eq!(f16::powi(f16::NAN, 0), 1.0);
+    /// ```
     #[inline]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f16", issue = "116909")]
@@ -347,8 +356,10 @@ impl f16 {
     ///
     /// let x = 2.0_f16;
     /// let abs_difference = (x.powf(2.0) - (x * x)).abs();
-    ///
     /// assert!(abs_difference <= f16::EPSILON);
+    ///
+    /// assert_eq!(f16::powf(1.0, f16::NAN), 1.0)
+    /// assert_eq!(f16::powf(f16::NAN, 0.0), 1.0)
     /// # }
     /// ```
     #[inline]