If documentation were adequate, there wouldn't have been a need for Stack Overflow in the first place. At least for now, it still takes (human) intelligence to consume documentation and synthesize knowledge through experience with new technologies, at which point it can be communicated to others. LLMs may get to that point, but they aren't there yet. Also, good documentation explains not only the what and the how of a codebase (which LLMs may be able to summarize from the code), but also the why and the when (e.g. when to use one method or another).