Clarify life cycle of RequestScoped

This is one of the areas where the CDI spec could be clearer. It defines
the points where "some" request scope is active, and where "some" request
scope is destroyed, but it doesn't clearly state what the span of a particular
request context is. You sort of have to work backwards and figure it out.

Update the spec to indicate when a particular request scope is
created, what operations it's active during, and when it's destroyed.