Add docblock documentation for class methods

client.php

@@ -10,28 +10,46 @@ public function __construct($url) {
     $this->api = new RestClient([ 'base_url' => rtrim($url, '/'), 'format' => 'json' ]);
   }
 
+  /**
+   * Create a new invoice.
+   *
+   * @param string|int $msatoshi
+   * @param object $metadata
+   * @return object the invoice
+   */
   public function invoice($msatoshi, $metadata=null) {
     $res = $this->api->post('/invoice',
       json_encode([ 'msatoshi' => $msatoshi, 'metadata' => $metadata ]),
       [ 'Content-Type' => 'application/json' ]);
 
-    if ($res->info->http_code !== 201)
-      throw new Exception('cannot create invoice');
+    if ($res->info->http_code !== 201) throw new Exception('saving invoice failed');
 
     return $res->decode_response();
   }
 
+  /**
+   * Fetch invoice by ID
+   *
+   * @param string $invoice_id
+   * @return object the invoice
+   */
   public function fetch($invoice_id) {
     $res = $this->api->get('/invoice/' . urlencode($invoice_id));
-    if ($res->info->http_code !== 200)
-      throw new Exception('unable to fetch invoice');
+    if ($res->info->http_code !== 200) throw new Exception('fetching invoice failed');
     return $res->decode_response();
   }
 
+  /**
+   * Register a new webhook.
+   *
+   * @param string $invoice_id
+   * @param string $url
+   * @return bool
+   */
   public function registerHook($invoice_id, $url) {
     $res = $this->api->post('/invoice/' . urlencode($invoice_id) . '/webhook', [ 'url' => $url ]);
     if ($res->info->http_code !== 201)
-      throw new Exception('unable to register hook');
+      throw new Exception('register hook failed');
     return true;
   }
 }