docs: add policy description.

Author: shy-1st

src/main/java/org/casbin/jcasbin/model/Policy.java

@@ -21,6 +21,9 @@
 import java.util.List;
 import java.util.Map;
 
+/**
+ * Policy represents the whole access control policy user defined.
+ */
 public class Policy {
     public Map<String, Map<String, Assertion>> model;