Clarify the behavior of what Boxstarter may do when it runs on a box
to install all the necessary tools so that there are no surprises to
the end user when the script is run.

Currently there is no interface that warns the user that Boxstarter
will reboot the machine possibly multiple times depending on how many
dependencies need to be installed and doesn't mention a need to disable
UAC. For folks who see what may look like a reboot loop, we feel it is
necessary to make them aware that UAC will be disabled and they will
need to take action to re-enable UAC manually if they interfere/stop
the script from finishing.

Fixes: nodejs/Release#369