I use three different add ons to write form submissions to a google doc or sheet. They are Form Publisher, Document Studio, and Autocrat. The only free version is autocrat, but over the years I have found it to be the buggiest and has the slowest, hardest to use interface.

• Form Publisher ($79) is an add-on to the Form, so the only data that can be merged is what is submitted on the form. For this use case, Form Publisher is the one that rewrites the form data to the SAME DOCUMENT so when you edit the form, the data flows to the document, which is therefore updated. All the changes that are made to the document are recorded in version history.
• Document Studio, ($29) in my opinion has the best interface, it runs in a sidebar and is very easy to work with. It is an add on to the sheet, so you can do fancy things on the form submission sheet like include vlookups, and such so that you can merge data that is not actually submitted on the form. DS also updates the document when you use the form edit URL, but it instead creates a new document with each edit. This will create a lot of versions of a project in the folder, however, it also can be setup to automatically merge the documents into a folder for each owner, so that the project documents are automatically filed according to owner (or any other field such as status).
• Autocrat (Free) is also an add on to the sheet, so like DS it can merge data that is not submitted. It has in my opinion the clumsiest interface, which forces you to go through the entire setup to get to one part you want to change. It is also the slowest interface to launch.