Clarify that session context is also destroyed at shutdown/undeploy

Section 6.7.2. states "The session context is destroyed
when the HTTPSession times out, after all HttpSessionListener s have been called, and at the very end of any request
in which invalidate() was called, after all filters and ServletRequestListener s have been called."

The first part of the above sentence should not only say that the context is destroyed after all listeners have been called specifically in the event of a session timeout. It should specify that this is also true when the session is being destroyed as a result of server shutdown or web-app undeploy.