Add docs about why X-Frame-Options is unavailable (#13902)

Author: hishamco

src/docs/reference/modules/Security/README.md

@@ -13,6 +13,9 @@ Enabling the `OrchardCore.Security` module will allow the user to set the follow
 | `PermissionsPolicy` | Gets or sets the `Permissions-Policy` HTTP header. |
 | `ReferrerPolicy` | Gets or sets the `Referrer-Policy` HTTP header. |
 
+!!! note
+    The `Content-Security-Policy` HTTP header contains the `frame-ancestors` directive which obsoleted the `X-Frame-Options` HTTP header.
+
 ## Security Settings Configuration
 
 The `OrchardCore.Security` module allows the user to use configuration values to override the `AdminSettings` by calling `ConfigureSecuritySettings()` extension method.