Update for 3.x and 4.x syntax

Right now the list of keywords is based on the [2.1 documentation](https://www.varnish-cache.org/docs/2.1/reference/vcl.html). 

Need to review the docs for [3.x](https://www.varnish-cache.org/docs/3.0/reference/vcl.html) and [4.x](https://www.varnish-cache.org/docs/4.0/reference/vcl.html) and incorporate anything new.
