Add some more information to the debug_workflow option and improve formatting

This MR adds some information to the documentation on the debug_workflow option (namely, as reported by Marco, that some things will run single-threaded when using Toil: see https://toil.readthedocs.io/en/latest/running/debugging.html#using-a-python-debugger for details). A check was also added to ensure that debug_workflow is used only when batch_system = single_machine. Some reformatting of the documentation was also done to make it more uniform.