auth_openidc.conf: record that default token binding policy is optional
# "optional": referred token binding will be requested, the "cnf["tbh"]" claim is optional on return
# "required": referred token binding will be requested, the "cnf["tbh"]" claim must be present when the Client supports Token Binding
# "enforced": referred token binding will be requested, the "cnf["tbh"]" claim must be present and the User Agent must support Token Binding
+# When not defined the default is "optional".
#OIDCTokenBindingPolicy [disabled|optional|required|enforced]
# (used only in dynamic client registration)
