Add note about usage of serf_debug__closed_conn by Subversion
(Subversion introduced this call in 2006)

* serf.h
  Add note in comment on a few debug helpers.


git-svn-id: https://svn.apache.org/repos/asf/serf/trunk@1812491 13f79535-47bb-0310-9956-ffa450edef68
1 file changed