docs(API): add note about the lifecycle of ui.item.sortable properties
docs(API): add note about the lifecycle of ui.item.sortable properties

Closes #353
MCKRUZ/ui-sortable,ballerabdude/ui-sortable,dtee/ui-sortable,MCKRUZ/ui-sortable,ballerabdude/ui-sortable,UmarMughal/ui-sortable,s001dxp/ui-sortable,zhaokunfay/ui-sortable,thgreasi/ui-sortable,looker/ui-sortable,dtee/ui-sortable,angular-ui/ui-sortable,UmarMughal/ui-sortable,looker/ui-sortable,zhaokunfay/ui-sortable,s001dxp/ui-sortable