Javadoc and xml documentation improvements

src/main/java/org/springframework/samples/petclinic/Pet.java

...main/java/org/springframework/samples/petclinic/Vets.java

...pringframework/samples/petclinic/web/CrashController.java

...pringframework/samples/petclinic/web/OwnerController.java

.../springframework/samples/petclinic/web/PetController.java

...ringframework/samples/petclinic/web/PetTypeFormatter.java

.../springframework/samples/petclinic/web/VetController.java

...pringframework/samples/petclinic/web/VisitController.java

...springframework/samples/petclinic/web/VisitsAtomView.java

src/main/webapp/WEB-INF/mvc-core-config.xml

...ngframework/samples/petclinic/web/VisitsAtomViewTest.java