Clarify diff two arrays instructions

- Clarify you want items that are in one but not both arrays
- Add "symmetric difference" terminology for those familiar with it

seed/challenges/01-front-end-development-certification/intermediate-bonfires.json

@@ -48,7 +48,7 @@
       "id": "a5de63ebea8dbee56860f4f2",
       "title": "Diff Two Arrays",
       "description": [
-        "Compare two arrays and return a new array with any items only found in one of the original arrays.",
+        "Compare two arrays and return a new array with any items only found in one of the two given arrays, but not both. In other words, return the symmetric difference of the two arrays.",
         "Remember to use <a href='//github.com/FreeCodeCamp/freecodecamp/wiki/How-to-get-help-when-you-get-stuck' target='_blank'>Read-Search-Ask</a> if you get stuck. Try to pair program. Write your own code."
       ],
       "challengeSeed": [