Clarify that the Context for RequestScoped must be active during @PreDestroy calls

We have the explicit rule that the Context for @RequestScoped must be active during @PostConstruct of any bean.
But it seems we don't force the same for invocations of @PreDestroy methods.

That's especially weird since a few containers now blow up during a destroyal of a @SessionScopedBean which has a @RequestScoped Principal injected, even if the session destroyal was triggered by an explicit Session#invalidate() call in an open HTTP request.