Improved and fixed twig reference

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | yes
| Applies to | 2.3+
| Ficket tickets | #4301 

Removed the ugly looking tables and added some more details.
are reference names global to the whole doc or only to the current file ? If they are global, it should at least be ``twig_function``