Specify that updating the secret name is not possible

Author: kbatuigas

modules/reference/pages/rpk/rpk-security/rpk-security-secret-update.adoc

@@ -41,7 +41,7 @@ rpk security secret update --name NETT2 --value value --scopes redpanda_connect,
 
 |-h, --help |- |Help for update.
 
-|--name |string |Name of the secret. The name must be in uppercase and can only contain letters, digits, and underscores.
+|--name |string |Name of the secret. The name must be in uppercase and can only contain letters, digits, and underscores. You cannot update the name of an existing secret.
 
 |--scopes |stringArray |Scope(s) of the secret (for example, `redpanda_connect`).