Add warning to stub/3 (#169)

Author: Paulo Daniel Gonzalez

lib/mox.ex

@@ -597,6 +597,16 @@ defmodule Mox do
       stub(MockWeatherAPI, :get_temp, fn _loc -> {:ok, 30} end)
 
   `stub/3` will overwrite any previous calls to `stub/3`.
+
+  > #### Warning {: .warning}
+  >
+  > `stub/3` does **not verify that a call occurred**.
+  > If the code stops calling the stubbed function
+  > (for example, after a refactor), the test will still pass.
+  >
+  > This can lead to unused stubs lingering in tests.
+  > Always use `expect/3` when you want to ensure the
+  > function is actually invoked.
   """
   @spec stub(mock, atom(), function()) :: mock when mock: t()
   def stub(mock, name, code)