HDDS-13801. [Docs] ozone s3 getsecret command and REST API supports user parameter

hadoop-hdds/docs/content/security/SecuringS3.md

@@ -37,18 +37,32 @@ The user needs to `kinit` first and once they have authenticated via kerberos
 
 ## Obtain Secrets
 
-* S3 clients can get the secret access id and user secret from OzoneManager.
+S3 clients can get the secret access id and user secret from OzoneManager.
 
+### Using the command line
+
+For a regular user to get their own secret:
 ```bash
 ozone s3 getsecret
 ```
 
-* Or by sending request to /secret S3 REST endpoint.
+An Ozone administrator can get a secret for a specific user by using the `-u` flag:
+```bash
+ozone s3 getsecret -u <username>
+```
+
+### Using the REST API
 
+A user can get their own secret by making a `PUT` request to the `/secret` endpoint:
 ```bash
 curl -X PUT --negotiate -u : https://localhost:9879/secret
 ```
 
+An Ozone administrator can get a secret for a specific user by appending the username to the path:
+```bash
+curl -X PUT --negotiate -u : https://localhost:9879/secret/<username>
+```
+
 This command will talk to ozone, validate the user via Kerberos and generate
 the AWS credentials. The values will be printed out on the screen. You can
 set these values up in your _.aws_ file for automatic access while working