docs(schedule): add waitForCompletion

docs for https://github.com/nestjs/schedule/pull/1870/files

Author: thomaschaaf

content/techniques/task-scheduling.md

@@ -144,6 +144,12 @@ Also, you can supply additional options as the second parameter to the `@Cron()`
         This allows you to specify the offset of your timezone rather than using the <code>timeZone</code> param.
       </td>
     </tr>
+    <tr>
+      <td><code>waitForCompletion</code></td>
+      <td>
+        If <code>true</code>, no additional instances of cronjob will run until the current onTick callback has completed. Any new scheduled executions that occur while the current cronjob is running will be skipped entirely.
+      </td>
+    </tr>
     <tr>
       <td><code>disabled</code></td>
       <td>