[SSL] Add note on private_key to BYOCA reqs partial (#22240)

Author: RebeccaTamachiro

src/content/partials/ssl/byo-ca-mtls-cert-requirements.mdx

@@ -30,3 +30,7 @@ import { Details } from "~/components"
 
 
 </Details>
+
+:::note
+Uploading the CA private key is only required if you wish to use [Zero Trust's block page](/cloudflare-one/connections/connect-devices/user-side-certificates/custom-certificate/). For more context on how mTLS works, refer to our [Learning Center](https://www.cloudflare.com/learning/access-management/what-is-mutual-tls/).
+:::