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
diff --git a/serf.h b/serf.h
index 5f3d3ae..9a504a4 100644
--- a/serf.h
+++ b/serf.h
@@ -1689,7 +1689,9 @@
 /** @} */
 
 
-/* Internal functions for bucket use and lifecycle tracking */
+/* Internal functions for bucket use and lifecycle tracking.
+   ### Some of these are directly or via Macros used by third party
+   ### applications, such as Apache Subversion */
 apr_status_t serf_debug__record_read(
     const serf_bucket_t *bucket,
     apr_status_t status);