TRUNK-3175,TRUNK-3176,TRUNK-3177 I added the javadocs for the ActiveItem...

List calss,ActiveItemType class and Allergy class. In addition to that i added javadocs for AllergySeverity class,AllergyType class,ProblemModifier class

https://tickets.openmrs.org/browse/TRUNK-3175
https://tickets.openmrs.org/browse/TRUNK-3177
https://tickets.openmrs.org/browse/TRUNK-3176