Cover edge cases for {f32, f64}.hypot() docs

Re-phrase in a way that handles input values being either 0 or
negative.

Author: jmaargh

library/std/src/f32.rs

@@ -581,8 +581,10 @@ impl f32 {
         unsafe { cmath::cbrtf(self) }
     }
 
-    /// Calculates the length of the hypotenuse of a right-angle triangle given
-    /// legs of length `x` and `y`.
+    /// Compute the distance between the origin and a point (`x`, `y`) on the
+    /// Euclidean plane. Equivalently, compute the length of the hypotenuse of a
+    /// right-angle triangle with other sides having length `x.abs()` and
+    /// `y.abs()`.
     ///
     /// # Examples
     ///

library/std/src/f64.rs

@@ -583,8 +583,10 @@ impl f64 {
         unsafe { cmath::cbrt(self) }
     }
 
-    /// Calculates the length of the hypotenuse of a right-angle triangle given
-    /// legs of length `x` and `y`.
+    /// Compute the distance between the origin and a point (`x`, `y`) on the
+    /// Euclidean plane. Equivalently, compute the length of the hypotenuse of a
+    /// right-angle triangle with other sides having length `x.abs()` and
+    /// `y.abs()`.
     ///
     /// # Examples
     ///