DOC: The option httplog is no longer valid in a backend.

This can be backported to 1.9 and 1.8.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e66bb22..e52235e 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2171,7 +2171,7 @@
 option http-use-htx                  (*)  X          X         X         X
 option httpchk                            X          -         X         X
 option httpclose                     (*)  X          X         X         X
-option httplog                            X          X         X         X
+option httplog                            X          X         X         -
 option http_proxy                    (*)  X          X         X         X
 option independent-streams           (*)  X          X         X         X
 option ldap-check                         X          -         X         X