A doc is not a 'word document' or an external file. It's a specific type of page on the website… You don't see it but it's right there. You posted your comment on it! Now… scroll up from where you posted this comment…. and there's your doc! It's called Test discussion document, and contains only one line that says:
"So here we can start a discussion, with reference links, attached files etc…"
Now, you go to create a doc, from the docs tab of whichever group you want your doc to be filed in, and there you go!