complex-numbers: Clarify that the exponent function isn't needed (#684)

Author: tasxatzial

exercises/practice/complex-numbers/.docs/instructions.append.md

@@ -0,0 +1,4 @@
+# Instructions append
+
+**Note:** The instructions ask for the exponent function because they are synced with a shared repository to maintain consistency across all language tracks.
+However, for this exercise in the Clojure track, you don't need to implement it.