[docs] TLS extended key usage notes (#6428)

libbeat/docs/shared-ssl-logstash-config.asciidoc

@@ -51,7 +51,7 @@ For more information about these configuration options, see <<configuration-ssl>
 * `ssl_certificate` and `ssl_key`: Specify the certificate and key that Logstash uses to authenticate with the client.
 * `ssl_verify_mode`: Specifies whether the Logstash server verifies the client certificate against the CA. You
 need to specify either `peer` or `force_peer` to make the server ask for the certificate and validate it. If you
-specify `force_peer`, and {beatname_uc} doesn't provide a certificate, the Logstash connection will be closed.
+specify `force_peer`, and {beatname_uc} doesn't provide a certificate, the Logstash connection will be closed. If you choose not to use {elasticsearch}/certutil.html[certutil], the certificates that you obtain must allow for both `clientAuth` and `serverAuth` if the extended key usage extension is present.
 +
 For example:
 +