varnish: add comment to avoid future pitfalls

Given we had to reconstruct how and why bits works in a quite different
way from other varnishes when troubleshooting the latest issues, we
decided to add a "big fat comment" here to clarify why things are in
place.

Change-Id: I18c53b80364c23b324326679dc5c84c16a958a10
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>