Added rustdoc for MultiFruit extract function (#1369)

src/collector/multi_collector.rs

@@ -87,6 +87,10 @@ pub struct FruitHandle<TFruit: Fruit> {
 }
 
 impl<TFruit: Fruit> FruitHandle<TFruit> {
+    // Extract a typed fruit off a multifruit.
+    //
+    // This function involves downcasting and can panic if the multifruit was
+    // created using faulty code.
     pub fn extract(self, fruits: &mut MultiFruit) -> TFruit {
         let boxed_fruit = fruits.sub_fruits[self.pos].take().expect("");
         *boxed_fruit