Explain amortized cost for Stack.push in documentation

src/Data/Stack.hs

@@ -95,7 +95,9 @@ clear s = writeSTRefU (tos s) 0
 
 -- | Insert a new element on top of the stack.
 --
--- Runtime: O(1)
+-- This will reallocate the underlying memory if needed.
+--
+-- Runtime: Amortized O(1)
 push :: VM.Unbox a => Stack s a -> a -> ST s ()
 push s x = do
   currentTos <- readSTRefU (tos s)