DOC: mention that "compression offload" is ignored in defaults section

This one is not inherited from defaults into frontends nor backends
because it would create a confusion situation where it would be hard
to disable it (since both frontend and backend would enable it).
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 96da1cc..d69bb11 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2158,7 +2158,8 @@
   invalid payloads. In this case, simply removing the header in the
   configuration does not work because it applies before the header is parsed,
   so that prevents haproxy from compressing. The "offload" setting should
-  then be used for such scenarios.
+  then be used for such scenarios. Note: for now, the "offload" setting is
+  ignored when set in a defaults section.
 
   Compression is disabled when:
     * the request does not advertise a supported compression algorithm in the