In theory, you should provide enough information for the user find the answer to her question, and no more. Inadequate support pages drive down customer loyalty (more details here), but complex pages encourage users to give up before finding an answer (more details here).
In practice, it’s very hard to make a support page both simple and comprehensive.
Let me suggest an alternative: context-sensitive Help. The idea is to integrate Help into your application such that when a user clicks the Help link she’ll be shown only the topics relevant to what she is currently doing. This will dramatically reduce the amount of information she has to sift through, and increase the probability of her finding an answer to her question.
Here’s an example:
Topics shown when on the Login page:
Topics shown when on the Settings page:
If context-sensitive Help is something you’re interested in then take a look at Convey - software that integrates into your web application and figures out the right Help topics to show your customers when they have a question. It only take 5 minutes to setup.
Full disclosure: I’m the founder of Convey