[varnish] add a few words about vmods
[varnish] add a few words about vmods

jetty-project/docker-docs,docker-library/docs,infosiftr/docker-library-docs,crate/docker-docs,Lightstreamer/docs,Bonitasoft-Community/docker_docs,docker-library/docs,crate/docker-docs,Bonitasoft-Community/docker_docs,infosiftr/docker-library-docs,Lightstreamer/docs,nats-io/docker-docs,nats-io/docker-docs,jetty-project/docker-docs