this is not exactly trivial, and i did not try it myself, so you have to take it as a "bet guess", but here goes:
there is no independent documentation for phpbb. the documentation you get here http://area51.phpbb.com/docs/code
is directly extracted from the source code itself, by analyzing it and the comments embedded in it.
this is done by a tool called phpDocumentor
lucky for you, both the tool and the raw material it operates on are open-source and are available.
phpbb source code is available in several formats (as a tarball, and through cvs), and so is the documentor.
you should install the documentor, get the source code on your local machine, and use the documentor to create a copy of the documentation you'll be able to browse locally.
hope it helps,