StackOverflow is good for:

• general questions (when you don't know where to look for) eg. how do I go about ...?

• specific questions (when you know what you want, in simple english) eg. suggest ways I can ...?

• quick fixes with more than one suggestion eg. I get this error, how to fix and please explain.

• understanding concepts as different people explain concepts differently eg. what is ...?

Documentation is good for:

• details (when you need to know more and when you really know what you need)

• features (find a list when you want to know what else you can do with it)

Few hours of surfing on stackoverflow can save you from 5 minutes of reading a documentation