The (currently) latest documentation for AUCTEX can be found here:
The directory contains the manual (auctex.pdf) and reference card (tex-ref.pdf). When a newer version is out by the time you read this, it can probably be found by clicking on Parent Directory and selecting the latest directory called something like ...
The Auctex LaTeX mode hook is LaTeX-mode-hook (note the capitalization). So try
(add-hook 'LaTeX-mode-hook (lambda() (setq comment-start "%% ")))
Note also that lambda expressions are self-evaluating so you don't need to quote them in this context.
I am assuming you are using M-; to comment things, otherwise comment-start does not enter into the ...