Make Btreeset::Insert docs more consistent

Author: bwinterton

src/libcollections/btree/set.rs

@@ -477,9 +477,9 @@ impl<T: Ord> BTreeSet<T> {
 
     /// Adds a value to the set.
     ///
-    /// If the set did not have a value present, `true` is returned.
+    /// If the set did not have this value present, `true` is returned.
     ///
-    /// If the set did have this key present, `false` is returned, and the
+    /// If the set did have this value present, `false` is returned, and the
     /// entry is not updated. See the [module-level documentation] for more.
     ///
     /// [module-level documentation]: index.html#insert-and-complex-keys