message: docs: describe callback param to Redoc.init
  fixes
diff: @@ -212,12 +212,13 @@ You can use all of the following options with standalone version on <redoc> tag
  ## Advanced usage of standalone version
  Instead of adding `spec-url` attribute to the `<redoc>` element you can initialize ReDoc via globally exposed `Redoc` object:
  ```js
  -Redoc.init(specOrSpecUrl, options, element)
  +Redoc.init(specOrSpecUrl, options, element, callback?)
  ```
  - `specOrSpecUrl` is either JSON object with specification or an URL to the spec in `JSON` or `YAML` format
  - `options` [options object](#redoc-options-object)
  - `element` DOM element to put ReDoc into
  +- `callback` (optional) - callback to be called after Redoc has been fully rendered
  ```js
  Redoc.init('http://petstore.swagger.io/v2/swagger.json', {
  