Clarify EL / CDI integration: implicit object or CDI producer for artifacts without required inject point support (request, session, component, cc)

It seems that in 2.3, we removed #{request}, when 2.3 mode is active and delegated to CDI.
Still available as "implicit object" AFAICS, when 2.3 mode is not active.

@arjantijms @BalusC any thought why it was not added?

