Add Details About username_rdn_name and username_rdn_oid to PKI Security Doc

deploy-manage/users-roles/cluster-or-deployment-auth/pki.md

@@ -44,7 +44,32 @@
     When you configure realms in `elasticsearch.yml`, only the realms you specify are used for authentication. If you also want to use the `native` or `file` realms, you must include them in the realm chain.
     ::::
 
-2. Optional: The username is defined by the [username_pattern](elasticsearch://reference/elasticsearch/configuration-reference/security-settings.md#ref-pki-settings). If you want to use something other than the CN of the Subject DN as the username, you can specify a regex to extract the desired username. The regex is applied on the Subject DN.
+2. Optional: If you want to use something other than the CN of the Subject DN as the username, you can use one of the following methods to extract the username:
+
+    * {applies_to}`stack: ga 9.1` Extract the username from a specific RDN attribute in the Subject DN.
+    * Using the [[username_pattern](elasticsearch://reference/elasticsearch/configuration-reference/security-settings.md#ref-pki-settings) setting, specify a regex to extract the desired username. The regex is applied on the Subject DN.
+
+    :::::{tab-set}
+
+   ::::{tab-item} Specific RDN attribute
+   The username can be extracted from a specific RDN attribute in the Subject DN by using [username_rdn_name](elasticsearch://reference/elasticsearch/configuration-reference/security-settings.md#ref-pki-settings) or [username_rdn_oid](elasticsearch://reference/elasticsearch/configuration-reference/security-settings.md#ref-pki-settings). When an RDN attribute configuration is provided, it supersedes `username_pattern`.
+
+   For example, to extract the username from the `CN` RDN attribute:
+
+   ```yaml
+   xpack:
+     security:
+       authc:
+         realms:
+           pki:
+             pki1:
+               order: 1
+               username_rdn_name: "CN"
+   ```
+   ::::
+
+    ::::{tab-item} Regex
+    Specify a regex to extract the desired username. The regex is applied on the Subject DN.
 
     For example, the regex in the following configuration extracts the email address from the Subject DN:
 
@@ -58,11 +83,13 @@
                 order: 1
                 username_pattern: "EMAILADDRESS=(.*?)(?:,|$)"
     ```
-
-    ::::{note}
+    :::{note}
     If the regex is too restrictive and does not match the Subject DN of the client’s certificate, then the realm does not authenticate the certificate.
+    :::
     ::::
 
+    :::::
+
 3. Optional: If you want the same users to also be authenticated using certificates when they connect to {{kib}}, you must configure the {{es}} PKI realm to allow delegation. See [PKI authentication for clients connecting to {{kib}}](#pki-realm-for-proxied-clients).
 4. Restart {{es}} because realm configuration is not reloaded automatically. If you’re following through with the next steps, you might wish to hold the restart for last.
 5. If you're using a self-managed cluster, then [enable SSL/TLS](../../security/secure-cluster-communications.md#encrypt-internode-communication).