Add init function to description of default extension points.

[kohkimakimoto]

Description

Add init function to description of default extension points in the extension document.
The relevant code can be found at the following:

htmx/src/htmx.js

Line 3741 in 145627a

init: function(api) {return null;},

Testing

This PR has just changed the document. There is no need for testing.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded. This PR has just changed the document. There is no need for testing.