Modify rule S115: explain that CONST_CASE applies to String, primitiv…

…es, and boxed types.

rules/S115/java/rule.adoc

@@ -57,6 +57,23 @@ public enum MyEnum {
 }
 ----
 
+=== Exceptions
+
+The rule applies to fields of primitive types (for example, ``float``), boxed primitives (``Float``), and Strings.
+We do not apply it to other types, which can be mutated, or have methods with side effects.
+
+[source,java]
+----
+public static final Logger log = getLogger(MyClass.class);
+public static final List<Integer> myList = new ArrayList<>();
+
+// call with side-effects
+log.info("message")
+
+// mutating an object
+myList.add(28);
+----
+
 == Resources
 
 === External coding guidelines