Linking to the manual/documentation is the correct approach, if as you said, the docs in question do indeed provide a sufficient answer to the question. This lends an "official" feel to your answer, and also gives the reader an obvious place to learn more about the topic, if they are so inclined.
However, when I see questions that appear to be so simple that they could be answered by reading the relevant documentation, I actually like to assume that the person asking the question has already read the manual, but didn't understand it. Copying and pasting the relevant sections out of the reference won't actually do the asker any good in and of itself, because they weren't sure what was meant.
Given that assumption, I always try to summarize what the manual actually says, in plain English. This has a couple of important benefits:
The obvious one. It explains what the documentation says in simple terms, rather than complicated techno-babble.
It helps to stave off link rot. In the case that the documentation were to go offline (either for technical reasons, obsolescence, or what have you), my answer would still be a useful and official response to the question that was posed.
It avoids having to copy and paste extremely long sections from the manual.
It seems to take advantage of the unique characteristic of this site that real people answer your questions, rather than machines. Google could have referred them to the documentation, but let's see Google do this! (Wait, I take that back. Probably best not to challenge Google...)