Box: add docs to point out contains() is more efficient than get()

Author: greenrobot

objectbox-java/src/main/java/io/objectbox/Box.java

@@ -319,6 +319,7 @@ public List<T> getAll() {
 
     /**
      * Check if an object with the given ID exists in the database.
+     * This is more efficient than a {@link #get(long)} and comparing against null.
      * @since 2.7
      * @return true if a object with the given ID was found, false otherwise
      */