Add return values for Application API methods

[skirtles-code]

Fixes #423.

There didn't seem to be an established convention for what form these should take. I had a look at both the Vue 2 and Vue 3 docs before making some arbitrary choices:

1. No name or type. In other contexts I might have included the type but for these methods I didn't think {Object} really added anything.
2. The values are a sub-list within Returns, even though they typically only have a single list item.
3. For the complicated cases (component and directive) I went for two items in the list rather than trying to cram it all into a single item.
4. I used a capital letter at the start of each item but no full-stop at the end. My reasoning was that these aren't proper sentences, they're more like items on a shopping list or a to-do list.

I made a couple of other small changes, both of which are directly related to these return values.

[NataliaTepluhina]

Thank you for this contribution @skirtles-code! Changes look good to me 👍🏻