Clarify that stagger transitions trigger only when data changes

[thelucre]

From the docs, the assumption is that a stagger attribute could/should work when using v-if or v-show, but staggered transitions are only triggered when the data supplied to the v-for is updated. A online liner would suffice in this section: http://vuejs.org/guide/transitions.html#Staggering-Transitions

See: vuejs/vue#3658 (comment)