DOC: config: Update doc for PR/PH session states to warn about rewrite failures

When an HTTP header rewrite failure is triggered, and 500-internal-error
response is returned. A "PR" termination state is logged if the error
occurred on the request and "PH" if the error is reported for the response.

The documentation was updated accordingly.

This patch is related to issue #1597.

(cherry picked from commit 24dda9403a7f1ca4a7425ead8da4e3ba4a0cd131)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 054da8c6a28d2b1fea90a718db13a892dadd43ec)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 00bc86a..3056ea0 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -21268,12 +21268,18 @@
           rare, that the proxy blocked a chunked-encoding request from the
           client due to an invalid syntax, before the server responded. In this
           case, an HTTP 400 error is sent to the client and reported in the
-          logs.
+          logs. Finally, it may be due to an HTTP header rewrite failure on the
+          response. In this case, an HTTP 500 error is sent (see
+          "tune.maxrewrite" and "http-response strict-mode" for more
+          inforomation).
 
      PR   The proxy blocked the client's HTTP request, either because of an
           invalid HTTP syntax, in which case it returned an HTTP 400 error to
           the client, or because a deny filter matched, in which case it
-          returned an HTTP 403 error.
+          returned an HTTP 403 error.  It may also be due to an HTTP header
+          rewrite failure on the request. In this case, an HTTP 500 error is
+          sent (see "tune.maxrewrite" and "http-request strict-mode" for more
+          inforomation).
 
      PT   The proxy blocked the client's request and has tarpitted its
           connection before returning it a 500 server error. Nothing was sent