doc: notes about variable hierarchy
doc: notes about variable hierarchy

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