You're welcome!
The first document is set up as everyone with a google account and a link can see, the last one needs permission because only the user who start the discussion typing the first comment can delete that discussion. So, if a discussion is over, I can contact that person and tell them to delete the discussion. In this way the document doesn't risk to be full of obsolete comments.
Maybe I should have mention it, I'll add it to the OP.