Python bindings documentation issues

I went through the Python bindings docs on the website and catched two issues:

• The advocated variable -DPYTHON_INSTALL_LOCATION="user" is actually called DUNE_PYTHON_INSTALL_LOCATION
• The text advocates the use of a variable ADDITIONAL_PIP_PARAMS. However, to my knowledge, this variable does not exist. There exists a parameter of that name to dune_python_install_package, but there is no global switch. If such a switch is added, a consistent naming scheme would be DUNE_PYTHON_ADDITIONAL_PIP_PARAMS.
• The pip flag for upgrading is --upgrade not -upgrade