I agree, I had several dozen users prompting me for installation instructions though and rather than hash out a meaningful solution like you're alluding to, I just solved the "right now" problem.
I'm all for working up a Grand Unified Installation document and trimming out ssh2's installation.xml when the time comes.
FWIW, the PDO docs also do a lot of duplication. I don't have much time to look into this now, but I agree that it would be great to have a general "install" doc, and specific extensions can document their own quirks.