docs: minor addition to design doc
docs: minor addition to design doc

sapitts/moose,dschwen/moose,SudiptaBiswas/moose,andrsd/moose,milljm/moose,bwspenc/moose,SudiptaBiswas/moose,idaholab/moose,idaholab/moose,andrsd/moose,harterj/moose,laagesen/moose,nuclear-wizard/moose,andrsd/moose,idaholab/moose,bwspenc/moose,bwspenc/moose,laagesen/moose,dschwen/moose,milljm/moose,lindsayad/moose,milljm/moose,harterj/moose,jessecarterMOOSE/moose,jessecarterMOOSE/moose,dschwen/moose,laagesen/moose,harterj/moose,sapitts/moose,milljm/moose,SudiptaBiswas/moose,lindsayad/moose,lindsayad/moose,bwspenc/moose,nuclear-wizard/moose,laagesen/moose,harterj/moose,idaholab/moose,jessecarterMOOSE/moose,nuclear-wizard/moose,sapitts/moose,SudiptaBiswas/moose,lindsayad/moose,lindsayad/moose,sapitts/moose,jessecarterMOOSE/moose,idaholab/moose,bwspenc/moose,sapitts/moose,dschwen/moose,dschwen/moose,SudiptaBiswas/moose,harterj/moose,jessecarterMOOSE/moose,andrsd/moose,andrsd/moose,nuclear-wizard/moose,milljm/moose,laagesen/moose