[doc] [java] ImmutableField: extend description, fixes #2108

pmd-java/src/main/resources/category/java/design.xml

@@ -784,7 +784,10 @@ of Object-Oriented Systems. Springer, Berlin, 1 edition, October 2006. Page 80.
           externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_design.html#immutablefield">
         <description>
 Identifies private fields whose values never change once object initialization ends either in the declaration
-of the field or by a constructor.  This helps in converting existing classes to becoming immutable ones.
+of the field or by a constructor. This helps in converting existing classes to becoming immutable ones.
+Note that this rule does not enforce referenced object to be immutable itself. A class can still be mutable, even
+if all its member fields are declared final. This is referred to as shallow immutability. For more information on
+mutability, see Effective Java, 3rd Edition, Item 17: Minimize mutability.
         </description>
         <priority>3</priority>
         <example>