doc or demo problem on animation transition section.

[wxsms]

as the doc stat:

CSS animations are applied in the same way as CSS transitions, the difference being that v-enter-from is not removed immediately after the element is inserted, but on an animationend event.

however, if we look into the following demo after this paragraph, the v-enter-from did being removed immediately after the element is inserted.

ref: vuejs/docs-next-zh-cn#858