DOC: mention that Squid correctly responds 400 to PPv2 header

Amos reported that Squid builds 3.5.0.0_20140624 and 3.5.0.0_20140630
were confirmed to respond correctly here and that any version will do
the same.
(cherry picked from commit 9e1382002aa1ba12dcc637870befd077ff887aad)