I've always found that documentation is a skill on it's own. You need to learn how to say something efficiently while answering as many questions as you can. If it's too much people won't read it, if it's too little then it's not useful.

Also learn how to ask questions, you know your project inside and out, try to go to someones position of not knowing anything. I always find stuff that seems so simple at first but then later realize that most people wouldn't know it or connect the dots because they don't know the project.

Here are some resources I've looked at.

https://jacobian.org/writing/what-to-write/

http://ericholscher.com/blog/2014/feb/27/how-i-judge-documen...

https://www.braintreepayments.com/blog/api-where-to-begin/

http://warpspire.com/talks/documentation/