Oh hi. That was me. :)
I don't think it's really complicated. Where the documentation's license terms are incompatible with CC BY-SA 2.5 (this includes Wikipedia, by the way), we're limited to reasonable fair-use. In general, consider that the author of the original document chose a certain license, and that that should be respected. (Just as I would hope my answers here wouldn't be copied verbatim into a non CC BY-SA book.)
Generally, this means small quotes rather than whole copies. Where the text is wholly functional, like a list of command-line arguments, there's probably more latitude. If it's necessary to explain the point, probably a paragraph or two isn't out of line.
But it shouldn't be the bulk of the answer. In fact, I think that's a decent rule of thumb: if you're quoting more than you're writing, give it a second thought. That's not just for copyright / license compatibility, though — it's better for the site to have actual original answers, not just copies of existing documentation.