Add a hint to Return Early Pattern for Functions

This commit adds a hint as disccussed in the issue, to clarify,
the difference between `undefined` and `"undefined"`.

Tested locally.

seed/challenges/01-front-end-development-certification/basic-javascript.json

@@ -3025,7 +3025,8 @@
         "<blockquote>function myFun() {<br>  console.log(\"Hello\");<br>  return \"World\";<br>  console.log(\"byebye\")<br>}<br>myFun();</blockquote>",
         "The above outputs \"Hello\" to the console, returns \"World\", but <code>\"byebye\"</code> is never output, because the function exits at the <code>return</code> statement.",
         "<h4>Instructions</h4>",
-        "Modify the function <code>abTest</code> so that if <code>a</code> or <code>b</code> are less than <code>0</code> the function will immediately exit with a value of <code>undefined</code>."
+        "Modify the function <code>abTest</code> so that if <code>a</code> or <code>b</code> are less than <code>0</code> the function will immediately exit with a value of <code>undefined</code>.",
+        "<strong>Hint</strong><br>Remember that <a href='http://www.freecodecamp.com/challenges/understanding-uninitialized-variables' target='_blank'><code>undefined</code> is a keyword</a>, not a string."
       ],
       "releasedOn": "January 1, 2016",
       "challengeSeed": [