Added doc comments to SecretsManager ListSecrets code example; remove…

…d SecretsManage helloworld as it is a duplicate of the create-secret code example (#510)

aws/sdk/examples/secretsmanager/src/bin/list-secrets.rs

@@ -24,6 +24,13 @@ struct Opt {
     verbose: bool,
 }
 
+/// Lists the names of your secrets.
+/// # Arguments
+///
+/// * `[-d DEFAULT-REGION]` - The region in which the client is created.
+///    If not supplied, uses the value of the **AWS_DEFAULT_REGION** environment variable.
+///    If the environment variable is not set, defaults to **us-west-2**.
+/// * `[-v]` - Whether to display additional information.
 #[tokio::main]
 async fn main() {
     let Opt { region, verbose } = Opt::from_args();