Clarify variables shared between `base.cfg` and `sandbox.cfg`

[wbenoit26]

I think it's not intuitive that, for example, kernel_length is specified in base.cfg but using a different value requires changing sandbox.cfg. What can we do to make this clearer?

Maybe we comment out any shared variables in base.cfg and add a note directing users to set those values in the other config file. Or, we could just have one config file, but put all the variables that might be changed between BNS/BBH runs into their own section. And would it break anything to put the [luigi_TuneRemote] table into base.cfg and just let the pipeline call dictate what gets used?