[Fix] jsx-uses-react: mark fragment variables as used

[remcohaszing]

This allows to use the following syntax:

import { createElement, Fragment } from 'react';

function Component {
  return <></>;
}

Or more common in Preact:

import { Fragment, h } from 'preact';

function Component {
  return <></>;
}

The react.fragment setting already existed for other rules, but it was
undocumented.

[ljharb]

This is documenting an existing setting, and it's either patch or minor to add it to jsx-uses-react, but I think it's reasonable to call this a bug fix.