Add missing links to API docs

Author: n7studios

src/ConvertKit_API.php

@@ -593,6 +593,8 @@ public function get_subscriber_tags(int $subscriber_id)
      * List purchases.
      *
      * @param array<string, string> $options Request options.
+     * 
+     * @see https://developers.convertkit.com/#list-purchases
      *
      * @throws \InvalidArgumentException If the provided arguments are not of the expected type.
      *
@@ -614,6 +616,8 @@ public function list_purchases(array $options)
      * Retuns a specific purchase.
      *
      * @param integer $purchase_id Purchase ID.
+     * 
+     * @see https://developers.convertkit.com/#retrieve-a-specific-purchase
      *
      * @return false|object
      */
@@ -631,6 +635,8 @@ public function get_purchase(int $purchase_id)
      * Creates a purchase.
      *
      * @param array<string, string> $options Purchase data.
+     * 
+     * @see https://developers.convertkit.com/#create-a-purchase
      *
      * @throws \InvalidArgumentException If the provided arguments are not of the expected type.
      *