DOC: stats: document the comp_rsp stats column

This was forgotten in recent patch 5e16cbc3.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 223d479..362d2fa 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -11043,6 +11043,7 @@
  51. comp_in: number of HTTP response bytes fed to the compressor
  52. comp_out: number of HTTP response bytes emitted by the compressor
  53. comp_byp: number of bytes that bypassed the HTTP compressor (CPU/BW limit)
+ 54. comp_rsp: number of HTTP responses that were compressed
 
 
 9.2. Unix Socket commands