Kill the reference to --admission-control option

The `--admission-control` option has been replaced by two new options in
v1.10. This PR kills the last appearance of the old option in the doc.

Author: tengqm

docs/admin/extensible-admission-controllers.md

@@ -176,8 +176,12 @@ user._ If there is an error encountered when calling an external admission
 webhook, that request is ignored and will not be used to approve/deny the
 admission request.
 
-**Note:** The admission chain depends solely on the order of the
-`--admission-control` option passed to `kube-apiserver`.
+**Note** Before v1.10, the admission chain depends solely on the order of the
+`--admission-control` option passed to `kube-apiserver`. Starting from v1.10,
+the `--admission-control` option is replaced by the `--enable-admission-plugins`
+and the `--disable-admission-plugins` options where the order of plugins for
+these two options no longer matters.
+{. note}
 
 ### Enable external admission webhooks