DOC: fix originalto except clause on destination address

Fix the description of the except clause of the originalto option. The
destination address and not the source is compared with the except range
address to prevent the addition of the X-Original-To header.

This can be backported in every releases.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e6dce41..4315fb7 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -8770,10 +8770,11 @@
   Sometimes, a same HAProxy instance may be shared between a direct client
   access and a reverse-proxy access (for instance when an SSL reverse-proxy is
   used to decrypt HTTPS traffic). It is possible to disable the addition of the
-  header for a known source address or network by adding the "except" keyword
-  followed by the network address. In this case, any source IP matching the
-  network will not cause an addition of this header. Most common uses are with
-  private networks or 127.0.0.1.  IPv4 and IPv6 are both supported.
+  header for a known destination address or network by adding the "except"
+  keyword followed by the network address. In this case, any destination IP
+  matching the network will not cause an addition of this header. Most common
+  uses are with private networks or 127.0.0.1. IPv4 and IPv6 are both
+  supported.
 
   This option may be specified either in the frontend or in the backend. If at
   least one of them uses it, the header will be added. Note that the backend's