Add the expose option and expose function

[skirtles-code]

Vue 3.2 introduced defineExpose, the expose option, and the expose function in the setup context object. While defineExpose is documented, the other two are not. This PR adds some documentation for those two forms of expose.

RFC: https://github.com/vuejs/rfcs/blob/master/active-rfcs/0042-expose-api.md

While working on this I noticed that mount returns the internal instance, not the exposed instance. This made it trickier to create simple, complete examples. I'd have preferred the API example to use const vm = Vue.createApp(...).mount(...) to grab the public instance, but that doesn't work. Instead I've used examples that show how to use expose but without explicitly showing how to use the exposed properties. I've opened a PR to change this in vue-next: vuejs/core#4606. If that gets merged we could revisit those examples to make them a little clearer.