Clarify symmetric difference definition + examples

- Make definition of symmetric difference more precise
- Add in set examples in description to facilitate learning of the
  concept
- Put "symmetric difference" in `<dfn>` tags

challenges/01-front-end-development-certification/advanced-bonfires.json

@@ -81,8 +81,8 @@
       "id": "a3f503de51cf954ede28891d",
       "title": "Symmetric Difference",
       "description": [
-        "Create a function that takes two or more arrays and returns an array of the symmetric difference of the provided arrays.",
-        "The mathematical term symmetric difference refers to the elements in two sets that are in either the first or second set, but not in both.",
+        "Create a function that takes two or more arrays and returns an array of the <dfn>symmetric difference</dfn> (<code>&xutri;</code> or <code>&oplus;</code>) of the provided arrays.",
+        "Given two sets (for example set <code>A = {1, 2, 3}</code> and set <code>B = {2, 3, 4}</code>), the mathematical term \"symmetric difference\" of two sets is the set of elements which are in either of the two sets, but not in both (<code>A &xutri; B = C = {1, 4}</code>). For every additional symmetric difference you take (say on a set <code>D = {2, 3}</code>), you should get the set with elements which are in either of the two the sets but not both (<code>C &xutri; D = {1, 4} &xutri; {2, 3} = {1, 2, 3, 4}</code>).",
         "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": [