I have recently started working on a complex project and recently had a team meeting where a new feature was discussed. I am not happy with the current architecture, because the system is undocumented. However, I have to prove myself in coming months and, hence, continue delivering.
To prove my point that documenting helps, I need some proof of concept and I am thinking about implementing this feature with high-quality documentation which can capture user needs as well as design decisions taken from the developer's point of view. Basically, I want to make it such that anyone should be able to understand the system by having a look at that document including me anytime in future.
I am very new to this stuff, and I don't know how to document this kind of stuff, and I don't have time to go through different books.
Given the current situation, which document(s) can accomplish this purpose? And how do I do it?