Add note about delay in Pages access control

doc/user/project/pages/pages_access_control.md

@@ -38,7 +38,9 @@ For a demonstration, see [Pages access controls](https://www.youtube.com/watch?v
      - **Only project members**: Only project members are able to browse the website.
      - **Everyone with access**: Everyone, both logged into and logged out of GitLab, is able to browse the website, no matter their project membership.
 
-1. Click **Save changes**.
+1. Click **Save changes**. Note that your changes may not take effect immediately. GitLab Pages uses
+   a caching mechanism for efficiency. Your changes may not take effect until that cache is
+   invalidated, which usually takes less than a minute.
 
 The next time someone tries to access your website and the access control is
 enabled, they're presented with a page to sign into GitLab and verify they