How to Document Existing Functions and API Calls

A lot of times when I'm doing my live streaming, I'm creating stuff that's for no one's consumption but my own. On the live stream when I'm coding live, I will rarely write out documentation because it's slow to do and it's kind of boring to watch. And in the methods that I have created documentation for, boy, do I feel relieved about a year later when I'm going in and looking at it.