[spec] Pop dummy frame after Invocation (#1691)

Author: r1ru

document/core/exec/modules.rst

@@ -798,6 +798,10 @@ Once the function has returned, the following steps are executed:
 
 2. Pop :math:`\val_{\F{res}}^m` from the stack.
 
+3. Assert: due to :ref:`validation <valid-module>`, the frame :math:`F` is now on the top of the stack.
+
+4. Pop the frame :math:`F` from the stack.
+
 The values :math:`\val_{\F{res}}^m` are returned as the results of the invocation.
 
 .. math::