Clarify wording on f64::round() and f32::round()

"Round half-way cases" is a little confusing (it's a 'garden path
sentence' as it's not immediately clear whether round is an adjective
or verb).

Make this sentence longer and clearer.

Author: Wilfred

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

@@ -69,8 +69,8 @@ impl f32 {
         unsafe { intrinsics::ceilf32(self) }
     }
 
-    /// Returns the nearest integer to `self`. Round half-way cases away from
-    /// `0.0`.
+    /// Returns the nearest integer to `self`. If a value is half-way between two
+    /// integers, round away from `0.0`.
     ///
     /// # Examples
     ///

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

@@ -69,8 +69,8 @@ impl f64 {
         unsafe { intrinsics::ceilf64(self) }
     }
 
-    /// Returns the nearest integer to `self`. Round half-way cases away from
-    /// `0.0`.
+    /// Returns the nearest integer to `self`. If a value is half-way between two
+    /// integers, round away from `0.0`.
     ///
     /// # Examples
     ///