"Plus, I've found that forcing myself to write function documentation, and justify my decisions, has resulted in me putting more consideration into design."

This, this, and... this.

Sometimes, I step back after writing documentation and realise, this is a bunch of baloney. It could be much simpler, or this is a terrible decision! My point: Writing documentation is about expressing the function a second time -- the first time was code, the second time was natural language. Yeah, it's not a perfect 1:1 (see: the law in any developed country!), but it is a good heuristic.