Are you referring to this line of the documentation?

envkey_SECRET_KEY: ${{ secrets.SECRET_KEY }}

I suppose the documentation might be a bit ambiguous, as the name of the variable only matters for what you'll expect to see in your envfile. The envkey_ prefix is only there for the Action to be able to discover the variables you want to add to the file. I suppose it might we worth adding what that example Action would output.

To be clear for now, the Action in the docs will output a file called .env that contains:

DEBUG: false
SOME_API_KEY: "123456abcdef"
SECRET_KEY: "the_secret_from_the_repository_secrets"