Minor documentation updates to input.hotkey.js (#2721)

Credit to @SebastiaanKloos

modules/system/assets/ui/docs/input-hotkey.md

@@ -18,4 +18,12 @@ Allows keyboard shortcuts (hotkeys) to be bound to an element's click event.
         Press "Shift + R" on your keyboard
     </button>
 
+## Javascript API
 
+If you use a selector other than a button or a link, you will need to add the `hotkeyVisible` property to the hotkey config.
+
+    $('html').hotKey({
+        hotkey: 'ctrl+s, cmd+s',
+        hotkeyVisible: false,
+        callback: doSomething
+    });

modules/system/assets/ui/js/input.hotkey.js

@@ -6,7 +6,7 @@
  *
  * JavaScript API:
  *
- * $('html').hotKey({ hotkey: 'ctrl+s, cmd+s', callback: doSomething);
+ * $('html').hotKey({ hotkey: 'ctrl+s, cmd+s', hotkeyVisible: false, callback: doSomething });
  */
 +function ($) { "use strict";