I think this really depends on culture. If you target OS APIs or the libc, the documentation is stellar. You have several standards and then conceptual documentation and information about particular methods all with historic and current and implementation notes, then there is also an interactive hypertext system. I solve 80% of my questions with just looking at the official documentation, which is also installed on my computer. For the remaining I often try to use the WWW, but these are often so specific, that it is more successful to just read the code.

Once I step out of that ecosystem, I wonder how people even cope with the lack of good documentation.