Add comments and update callback arguments' names (#36082)

* Add comments and update callback arguments' names

I've added a few comments to the solution and given the callback arguments more descriptive names to clarify what the solution is doing, since this challenge seemed a little more complex than the previous challenges.

* Update guide/english/certifications/javascript-algorithms-and-data-structures/functional-programming/use-the-reduce-method-to-analyze-data/index.md

That makes absolute sense and is clearer. Thank you.

Co-Authored-By: Randell Dawson <5313213+RandellDawson@users.noreply.github.com>

* Update indentation to be two-spaced

guide/english/certifications/javascript-algorithms-and-data-structures/functional-programming/use-the-reduce-method-to-analyze-data/index.md

@@ -120,8 +120,15 @@ var watchList = [
 ];
 
 // Add your code below this line
-
-var averageRating = watchList.filter(x => x.Director === "Christopher Nolan").map(x => Number(x.imdbRating)).reduce((x1, x2) => x1 + x2) / watchList.filter(x => x.Director === "Christopher Nolan").length;
+var averageRating = watchList
+  // Use filter to find films directed by Christopher Nolan
+  .filter(film => film.Director === "Christopher Nolan")
+  // Use map to convert their ratings from strings to numbers
+  .map(film => Number(film.imdbRating))
+  // Use reduce to add together their ratings
+  .reduce((sumOfRatings, rating) => sumOfRatings + rating) 
+  // Divide by the number of Nolan films to get the average rating 
+  / watchList.filter(film => film.Director === "Christopher Nolan").length;
 
 // Add your code above this line