OI build docs need illumos-closed package

[rmustacc]

Right now to get the closed-bins on OI you need to install the illumos-closed package as well which is missing from the list of packages.

[jclulow]

Is that true? The instructions currently say (for OI) to:

sudo pkg install build-essential \
    runtime/python-35 \
    developer/gcc-7

Looking at the dependencies of build-essential, it really seems like it should pull in the illumos-closed package:

$ pkg contents -r -m build-essential | grep closed
depend fmri=developer/illumos-closed type=require

[rmustacc]

It did not for a user on IRC that I was working through this with.