docs: Update section about Fine-grained PATs

[emilbader]

Description

In https://github.com/renovatebot/github-action#token it's claimed that fine-grained PATs cannot be used since they don't support GitHub's GraphQL API. This seems to have been implemented now.

[viceice]

feel free to open a PR 🤗

[HonkingGoose]

Is this issue about the docs, or about getting renovatebot/github-action to work with fine-grained PATs on GitHub's GraphQL API? 😕

It sounds like the bug reporter wants us to get the action to work with GraphQL?

Related issue/PR

We had this issue:

• README should explicitely mention that Fine-grained PATs are not supported #777

That was closed with:

• docs: mention that Fine-grained PATs are not supported #778

Copy/paste of current readme

It looks like the readme properly mentions the problems with fine-grained tokens and the GitHub GraphQL API:

### `token`

[Generate a Personal Access Token (classic)](https://github.com/settings/tokens), with the `repo:public_repo` scope for only public repositories or the `repo` scope for public and private repositories, and add it to _Secrets_ (repository settings) as `RENOVATE_TOKEN`.
You can also create a token without a specific scope, which gives read-only access to public repositories, for testing.
This token is only used by Renovate, see the [token configuration](https://docs.renovatebot.com/self-hosted-configuration/#token), and gives it access to the repositories.
The name of the secret can be anything as long as it matches the argument given to the `token` option.

Note that Renovate _cannot_ currently use [Fine-grained Personal Access Tokens](https://github.com/settings/tokens?type=beta) since they do not support the GitHub GraphQL API, yet.

Note that the [`GITHUB_TOKEN`](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token#permissions-for-the-github_token) secret can't be used for authenticating Renovate because it has too restrictive permissions.
In particular, using the `GITHUB_TOKEN` to create a new `Pull Request` from more types of Github Workflows results in `Pull Requests` that [do not trigger your `Pull Request` and `Push` CI events](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow).

If you want to use the `github-actions` manager, you must setup a [special token](#special-token-requirements-when-using-the-github-actions-manager) with some requirements.