StackOverflow is good for:
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general questions (when you don't know where to look for) eg. how do I go about ...?
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specific questions (when you know what you want, in simple english) eg. suggest ways I can ...?
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quick fixes with more than one suggestion eg. I get this error, how to fix and please explain.
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understanding concepts as different people explain concepts differently eg. what is ...?
Documentation is good for:
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details (when you need to know more and when you really know what you need)
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features (find a list when you want to know what else you can do with it)