darts: link to explanations for math lingo (#1976)

* darts: link to explanations for math lingo

see also #1971 even if this change doesn't resolve the issue.

* Update exercises/darts/description.md

exercises/darts/description.md

@@ -14,4 +14,4 @@ In our particular instance of the game, the target rewards with 4 different amou
 
 The outer circle has a radius of 10 units (This is equivalent to the total radius for the entire target), the middle circle a radius of 5 units, and the inner circle a radius of 1. Of course, they are all centered to the same point (That is, the circles are [concentric](http://mathworld.wolfram.com/ConcentricCircles.html)) defined by the coordinates (0, 0).
 
-Write a function that given a point in the target (defined by its `real` cartesian coordinates `x` and `y`), returns the correct amount earned by a dart landing in that point.
+Write a function that given a point in the target (defined by its [cartesian coordinates](https://www.mathsisfun.com/data/cartesian-coordinates.html) `x` and `y`, where `x` and `y` are [real](https://www.mathsisfun.com/numbers/real-numbers.html)), returns the correct amount earned by a dart landing in that point.