You can build Unbound's documentation by running make doc in the source directory. This requires Doxygen to be installed on your system. If you have configured Unbound to be compiled with pyunbound (./configure --with-pyunbound) or with the python module (./configure --with-pythonmod) and you have Sphinx installed on your system, extra Python documentation will also be generated.
In case you have installed Unbound via your system's package manager, the man page should be installed alongside the application.
You can also find the latest documentation, along with tutorials and guides here.