The parameter name in the VirtualBox provider source is nic_type, not nictype.
The typo is probably caused by the fact that the parameter maps to nictype args in the VirtualBox CLI.
v2/networking/index.html.md encourages the reader to seek
provider-specific information under the documentation for the provider,
so for consistency, any virtualbox-specific networking info should be
placed there, not in the general networking section.