add javadoc to AreaCalculator

src/main/java/ru/lionarius/AreaCalculator.java

@@ -1,7 +1,17 @@
 package ru.lionarius;
 
+/**
+ * A class that provides methods to calculate the area of different geometric shapes.
+ */
 public class AreaCalculator {
 
+    /**
+     * Calculates the area of a circle given its radius.
+     *
+     * @param radius The radius of the circle.
+     * @return The area of the circle.
+     * @throws IllegalArgumentException if the radius is negative.
+     */
     public double circle(double radius) {
         if (radius < 0.0)
             throw new IllegalArgumentException("radius cannot be negative");