NO! You can't force them, it's not their job. It is your job (or project manager's job).
I am a software developer, and if somebody told me to document the process of how the system works I wouldn't do it.
(Actually I have been asked many times and told them to shove it, resigned or had a massive fallout - Because I knew the proper way it should be done without cutting corners and costs)
That is the job of the project manager, to deliver the specification to me, so I know how to do MY job, signed off by the big bosses. So that later it's not, why did you this and I thought you would do that. Bugger that.
What you are experiencing is a very common problem, where somebody wrote some code to do something and it turned into a business, dogs arse forward (in reverse). The developer wrote code based on what the boss said and now the project is so big nobody knows nothing.
The solutions is:
- Source Control (TFS or GitHub)
- Ticketing system. Jira, TFS tasking, Kanbanary, whatever.
- Central drive with documents or online Wiki
Project manager creates documentation to accompany the ticket/task, and makes sure it is updated in the main specification document. (Usually Business Analyst does this.. but lack of BA responsibly falls on PM)
Project manager manages it all, developers check in work and pick tickets or PM assigns tickets to dev if the devs are "tools" and can't be responsible and pick tickets themselves.