Add support to admin framework for retrieving an i18n description of the constraints applicable to an aggregation property. This is required for issue 2004 (HTML configuration guide generation).

Two new methods are added to the AggregationPropertyDefinition: getSourceConstraintSynopsis() and getSourceConstraintSynopsis(Locale). All aggregation definitions which had constraints have been updated to included the description. In addition, dsconfig is modified to output the constraint description when displaying verbose help on aggregation properties.