I have two answers for this question:
1- 3-30-300 second of finding an information (how to deal with information that can grow)
Showing all the functions with a search bar is a must (clustering them can make it easier).
Secondly these findings should be explained in a summarized way. If user wants to take more information and details about the same function:he can go into deep.
A header and main description should be visible at first. An extended example and related functions come second. And later on, detailed expert functions and explanations should come.
Side bar on the right is the first visible one and it should provide and overall view like the same concept that is used in Operation system help modules and pdf index function.
2- UX starts way before from this point:
UX does not start with the navigation. This answer has more about the style of education and documentation. If you want to know why they are listing and and later on explanation.
When I was learning English in my first years, my teacher suggested me to read dictionary and learn the meanings of the words from a list. One year later, I met with another teacher who was really helped me to learn English. She suggested me to use what i learned in an abstract conversation. When I compared these two approaches, I felt more confident in the second one.
Seeing the results of using something can be done much more faster and using information makes you learn it easier (not always).
If a visitor is reading your documentation;
- Want to see if it is suitable (potential customers)
- Want to see how it will be implemented (potential customers)
- Need more information (returning customer)
API Documentation should not be perceived just all about point 3. It should not be all about functions and what these functions are coming from and where it goes to. It should create some space for playing around with it.
Practicability is a crucial for converting minds and showing the possibilities of the solution. This strategy is working quite good for several sites: www.angularjs.org as I remember one of the first one took this approach and is currently being in use by many developers. Www.codeacademy.org, www.mashape.com also build business around the same idea.
It is good to look at this topic: http://en.wikipedia.org/wiki/Experiential_learning
Hope that it helps.
PS: The target audience is competent developers and enthusiastic amateur developers. Their demand may be different.