update: doc comment

src/libcore/slice/mod.rs

@@ -2668,7 +2668,9 @@ pub fn is_sorted_by_key<F, K>(&self, f: F) -> bool
     /// such that all those that return true precede the index and
     /// such that all those that return false succeed the index.
     ///
-    /// 'self' must be partitioned.
+    /// The slice must be partitioned
+    /// so that all elements where the predicate returns true
+    /// precede the elements where the predicate returns false.
     ///
     /// # Examples
     ///