As of b63, the fix done in the OpenMBeanOperationInfoSupport javadoc for the 6320104 is not correct.
The new javadoc for the 2 constructors contains:

[...]
impact - can be only one of ACTION, ACTION_INFO or INFO.
[...]
IllegalArgumentException - if name or description are null or empty string, or returnOpenType is null, or impact is not one of ACTION, ACTION_INFO, INFO, or UNKNOWN.
[...]

The first sentence continues to explicitely forbid UNKNOWN as a valid impact. Only the condition when IAE is thrown considers UNKNOWN as valid.
The two sentences contradict each other.