feat: added created hook to directives

[NataliaTepluhina]

Description of Problem

Current documentation misses the created custom directives hook added in vuejs/core@11804fe

Proposed Solution

I've added a new hook to API, Migration guide and Guide

/close #430

[skirtles-code]

I've added some thoughts to a couple of lines.

As an aside, it seems I'm to blame for adding a lot of unnecessary spaces at the ends of lines. My text editor is faithfully respecting the settings in .editorconfig, which includes the following:

[*.md]
trim_trailing_whitespace = false

I understand why it's there but it is causing these extra spaces in some of my contributions. Locally I've now hacked that file so, hopefully, any future changes will be trimmed.

[skirtles-code]

Sorry for the delay. This has been buzzing around at the back of my mind all week but I wasn't able to think of a way to write it that I'm entirely happy with.

I've made some concrete suggestions this time. I've removed any mention of the parent component or props and have instead used the phrase 'element's attributes or event listeners'. I think that's an accurate description and conveys it in a way that will be relatable to most readers.