Comment # 3 on bug 1210865 from Daniel Mach

From `osc build --help`:

  --buildtool-opt OPT   pass option OPT to the build tool command (rpmbuild),
for example:
                          * don't clean build environment after a successful
build:
                            --buildtool-opt=--noclean

I think it's sufficiently documented.
As I explained in the issue that requested this, I don't want to add an option
for everything, it's better to document commonly used options this way.