As a PM, I created internal and external documentation for my features. It was used for product marketing, customer support, API documentation, and training.
I start writing documentation by using empathy and qualitative research to figure out what questions a user will have about the feature. This is a great framework even if I'm not writing a FAQ. Their first question is usually, "How will this make my life better?"
My features were complex, so it was a challenge to make them understandable to users with different levels of expertise.