docs: document removeAt() return value and update list examples

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