You can inject the Security Definitions widget anywhere in your specification description:
...
## Authorization
Some description
<!-- Redoc-Inject: <security-definitions> -->
...The inject instruction is wrapped in an HTML comment, so it is visible only in Redoc and not visible, for instance, in the SwaggerUI.
If the injection tag is not found in the description, it is appended to the end
of description under the Authentication header.
If the Authentication header is already present in the description,
Security Definitions are not inserted or rendered.