Document how to add conditional dependencies (#14694)
Document how to add conditional dependencies (#14694)

* add short docs section on conditional dependencies
* add reference to spec syntax
* add note that conditional dependencies can save time

LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack