What info does this look for in an action repo?

[o-az]

With some actions I don't get autocomplete and I'm wondering what info/files does an action need to have in its repo for this extension to provide autocomplete.

This is one example where I don't get autocomplete and also get errors because the extension isn't recognizing the action at all:

https://github.com/serverless/github-action

Is it because the action doesn't have action.yml file in its repo, if not then what's the reason so I can make a PR and add the necessary files/ info?