fvkernels: add comments explaining compilcated residual condition logic
fvkernels: add comments explaining compilcated residual condition logic

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