EDIT: I noticed that this answer actually has the same format as @WilliamAnderson's answer. That was completely coincidental. Either that, or I thought of these two answers after reading the previously mentioned dev's answer. I'm sorry
MDN's voice-change-o-matic has a help button that when clicked causes an information panel to slide into view. This panel has extra info on certain APIs used to make the application. The APIs are of course linked to documentation pages. Here's the example in action:
You can use this idea to create a help button that can even show all the documentation. But, of course, this brings up the issue of information overload. To go around this human issue, you can do what Google does, have a documentation only search bar. This bar will allow your users to search for what they want and then show them the things related to that search. An example would be Google Analytics, as you will see below, it helps a lot to have a search bar that will show you the article/documentation.
By the way, these articles aren't specifically created for the help bar, but instead it loads the Google support articles for Google Analytics
MDN also uses tooltips in an interesting way: the link has a very descriptive tool tip of the API being talked about (example)