This has minor cleanup of the documentation on VirtualBox
drivers. It fixes some grammatical issues and adds a bit more
clarification to some of the instructions for setting up a
developer's environment.
This is a follow up to I2018f9a89a3644863569ddac3126db71b125fb59.
Change-Id: I0f84554c98317160bfdfee14ba923cff4adda773
Implements: blueprint ironic-virtualbox-webservice-support
ccbfdeee8d