Document removeAt() return value. Fix #176.

doc/site/core/list.markdown

@@ -37,6 +37,10 @@ are shifted up to fill in where the removed element was.
     list.removeAt(1)
     IO.print(list) // "[a, c, d]".
 
+Returns the removed item.
+
+    IO.print(["a", "b", "c"].removeAt(1) // "b".
+
 It is a runtime error if the index is not an integer or is out of bounds.
 
 ### **[**index**]** operator

doc/site/lists.markdown

@@ -95,9 +95,13 @@ given position in the list. All following items are shifted up to fill in the
 gap:
 
     :::dart
-    var letters = ["a", "b", "c"]
+    var letters = ["a", "b", "c", "d"]
     letters.removeAt(1)
-    IO.print(letters) // ["a", "c"]
+    IO.print(letters) // ["a", "c", "d"]
+
+The `removeAt` method returns the removed item:
+
+    IO.print(letters.removeAt(1)) // "c"
 
 If you want to remove everything from the list, you can clear it: