MINOR: debug: add a special converter which display its input sample content.

This converter displays its input sample type and content. It is useful
for debugging some complex configurations.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 59425aa..163e873 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -10930,6 +10930,11 @@
 	default_backend servers
 	http-request set-header X-DeviceAtlas-Data %[req.fhdr(User-Agent),da-csv(primaryHardwareType,osName,osVersion,browserName,browserVersion)]
 
+debug
+  This converter is used as debug tool. It dumps on screen the content and the
+  type of the input sample. The sample is returned as is on its output. This
+  converter only exists when haproxy was built with debugging enabled.
+
 div(<value>)
   Divides the input value of type unsigned integer by <value>, and returns the
   result as an unsigned integer. If <value> is null, the largest unsigned