message: Add mention about default_value lifetime constraint
diff: @@ -94,7 +94,10 @@ class AttributeSpec {
  // jni_type: the type of this attribute in JNI layer (see OperationBuilder)
  // description: a description of this attribute, in javadoc
  // iterable: true if this attribute is a list
  - // default_value: default value for this attribute or nullptr if none
  + // default_value: default value for this attribute or nullptr if none. Any
  + // value referenced by this pointer must outlive the lifetime
  + // of the AttributeSpec. This is guaranteed if the value is
  + // issued by an OpDef of the global OpRegistry.
  AttributeSpec(const string& op_def_name, const Variable& var,
  const Type& type, const Type& jni_type,
  const string& description, bool iterable,
  