[DOC] document HTTP status codes

doc/configuration.txt

@@ -515,6 +515,29 @@ found there, but it's a common practice to respect the well-established
 messages. It can be composed of one or multiple words, such as "OK", "Found",
 or "Authentication Required".
 
+Haproxy may emit the following status codes by itself :
+
+  Code  When / reason
+   200  access to stats page, and when replying to monitoring requests
+   301  when performing a redirection, depending on the configured code
+   302  when performing a redirection, depending on the configured code
+   303  when performing a redirection, depending on the configured code
+   400  for an invalid or too large request
+   401  when an authentication is required to perform the action (when
+        accessing the stats page)
+   403  when a request is forbidden by a "block" ACL or "reqdeny" filter
+   408  when the request timeout strikes before the request is complete
+   500  when haproxy encounters an unrecoverable internal error, such as a
+        memory allocation failure, which should never happen
+   502  when the server returns an empty, invalid or incomplete response, or
+        when an "rspdeny" filter blocks the response.
+   503  when no server was available to handle the request, or in response to
+        monitoring requests which match the "monitor fail" condition
+   504  when the response timeout strikes before the server responds
+
+The error 4xx and 5xx codes above may be customized (see "errorloc" in section
+2.2).
+
 
 2.1.3.2) The response headers
 -----------------------------