I'm a technical writer, and I can confirm that it's really hard to find a balance between simple basics and deep technical details. Really, it comes down to a decision about how to give the most benefit to the largest number of users.

For most end-user documentation, you can think of the documentation as a boat and the users as people with different levels of swimming skills:

• The expert users are already excellent swimmers who don't really need our help, or who want to go diving instead of swimming on the surface. They'll be OK if we just leave them in the water. If they run into trouble, they can climb into the boat on their own.
• The beginner users who can't swim at all need more help than we can give them. If a swimming class (product training) is available, we can try to get them into it to learn the basics before they venture into open water. If there are lifeguards (technical support), they can jump in to rescue them.
• The intermediate users are adequate swimmers, but they might not have gone swimming in this area before. We can bring them into the boat to help them navigate the trickier sections of the waterway. If they feel like they're ready for something more advanced, they can jump out and explore the depths on their own.