Adding javadoc for removal listener

Signed-off-by: Sagar Upadhyaya <sagar.upadhyaya.121@gmail.com>

server/src/main/java/org/opensearch/common/cache/RemovalListener.java

@@ -42,5 +42,10 @@
 @ExperimentalApi
 @FunctionalInterface
 public interface RemovalListener<K, V> {
+
+    /**
+     * This may be called from multiple threads at once. So implementation needs to be thread safe.
+     * @param notification removal notification for desired entry.
+     */
     void onRemoval(RemovalNotification<K, V> notification);
 }