docs: add Javadoc to FibonacciSeries (#7215)

* docs: add Javadoc to FibonacciSeries

* fix: make small adjustment

---------

Co-authored-by: Deniz Altunkapan <deniz.altunkapan@outlook.com>

src/main/java/com/thealgorithms/recursion/FibonacciSeries.java

@@ -1,16 +1,26 @@
 package com.thealgorithms.recursion;
 
-/*
-    The Fibonacci series is a sequence of numbers where each number is the sum of the two preceding ones,
-    starting with 0 and 1.
-       NUMBER    0 1 2 3 4 5 6 7  8  9  10 ...
-       FIBONACCI 0 1 1 2 3 5 8 13 21 34 55 ...
-*/
+/**
+ * The Fibonacci series is a sequence of numbers where each number is the sum of the two preceding ones,
+ * starting with 0 and 1.
+ * <p>
+ * Example:
+ * 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 ...
+ * </p>
+ */
 
 public final class FibonacciSeries {
     private FibonacciSeries() {
         throw new UnsupportedOperationException("Utility class");
     }
+
+    /**
+     * Calculates the nth term in the Fibonacci sequence using recursion.
+     *
+     * @param n the position in the Fibonacci sequence (must be non-negative)
+     * @return the nth Fibonacci number
+     * @throws IllegalArgumentException if n is negative
+     */
     public static int fibonacci(int n) {
         if (n < 0) {
             throw new IllegalArgumentException("n must be a non-negative integer");