Add doc reference for client_auth_method

Support for additional Client authentication methods was added in
the OIDC realm in elastic#58708. This change adds the `rp.client_auth_method`
in the realm settings reference doc.

Author: jkakavas

docs/reference/settings/security-settings.asciidoc

@@ -1633,6 +1633,14 @@ at the OpenID Connect Provider.
 The OAuth 2.0 Client Secret that was assigned to {es} during registration
 at the OpenID Connect Provider.
 
+// tag::rp-client-auth-method-tag[]
+`rp.client_auth_method` {ess-icon}::
+(<<static-cluster-setting, Static>>)
+The Client authentication method used by elasticsearch as a client to authenticate
+to the OpenID Connect Provider. Can be `client_secret_basic`, `client_secret_post`,
+or `client_secret_jwt`. Defaults to `client_secret_basic`.
+// end::rp-client-auth-method-tag[]
+
 // tag::rp-redirect-uri-tag[]
 `rp.redirect_uri` {ess-icon}::
 (<<static-cluster-setting,Static>>)