improve docs regarding builtin.vcl

[nigoroll]

https://github.com/varnishcache/varnish-cache/blob/master/doc/sphinx/users-guide/vcl-built-in-code.rst might need an update.
During bugwash, consensus was that the purpose of "extensible" builtin subs should be explained in builtin.vcl. The rst referenced above should probably explain the concepts and reference these comments.

#3994 could make some steps in this direction