DOC: document option http-ignore-probes

This one was forgotten.
(cherry picked from commit 82649f9ef351a80c0859960208fc656d43b224cb)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 1923515..340b133 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1350,6 +1350,7 @@
 option forceclose                    (*)  X          X         X         X
 -- keyword -------------------------- defaults - frontend - listen -- backend -
 option forwardfor                         X          X         X         X
+option http-ignore-probes            (*)  X          X         X         -
 option http-keep-alive               (*)  X          X         X         X
 option http-no-delay                 (*)  X          X         X         X
 option http-pretend-keepalive        (*)  X          X         X         X