Document cml send-github-check authentication

Author: 0x2b3bfa0

content/docs/ref/send-github-check.md

@@ -7,6 +7,8 @@ cml send-github-check [options] <markdown report file>
 Similar to [`send-comment`](/doc/ref/send-comment), but using GitHub's
 [checks interface](https://docs.github.com/en/rest/reference/checks).
 
+ⓘ Authentication must be done through an automatically generated [`GITHUB_TOKEN`](https://docs.github.com/en/actions/security-guides/automatic-token-authentication) or a [GitHub App token](https://docs.github.com/en/developers/apps/building-github-apps/authenticating-with-github-apps); personal access tokens aren't allowed to create checks.
+
 ## Options
 
 Any [generic option](/doc/ref) in addition to: