Perhaps the searches are [EDIT: not] looking in the PDFs? I just opened the Summary for Policymakers for the Physical Science Basis report, and found a few discussions of water vapor in the atmosphere.

I'd try looking at "Physical Science Basis" and "Impacts, Adaptation and Vulnerability" reports; try the Summaries for Policymakers first, and the full reports if those don't have what you need (though go straight to the full reports if you have time -- they can be fascinating!).

I hope that helps.