Describe period=0 behavior

Author: Daisuke Baba

src/locales/en-US/generic-ble.html

@@ -41,6 +41,7 @@ <h3>Inputs</h3>
 </pre>
 When you set the above in <code>msg.payload</code>, <code>msg.topic</code> is ignored.
 The <code>period</code> property is the period for subscribing <code>Notify</code> events in milliseconds <b>per characteristic</b>.
+<code>0</code> or less value means to keep subscribing.
 </dd>
 </dl>
 </p>