You will need:
The API documentation is generated from source code comments (for now, just qdoc3 tool is supported, doxygen support will be added soon).
qdoc3 is the tool used to generate the oficial Qt4 documentation, you will use it to generate a bunch of XML files that will be used by the generator to create the documentation.
You need to tell PySide where it can find the Qt source code, to do this, when running cmake add the following parameters:
$ mkdir build
$ cmake -DQT_SRC_DIR=PATH_TO_QT_SOURCE_DIR ..
$ make apidoc
$ make apidocinstall
The documentation will be installed at <INSTALL_PREFIX>/share/doc/pyside/index.html, change the CMAKE_INSTALL_PREFIX value if you want to install the documentation in another directory.
If you want to have inheritance graphs on the generated documentation, make sure you have the bindings installed before generate the documentation.