Minor typos and NodeResourceGroup clarification
Minor typos and NodeResourceGroup clarification

Fix couple of typos and clarify the documentation about the NodeResourceGroup. 
Better documentation from the feedback in: https://github.com/kubernetes/autoscaler/issues/884
mmerrill3/autoscaler,bskiba/autoscaler,bskiba/autoscaler,kubernetes/autoscaler,kgrygiel/autoscaler,mmerrill3/autoscaler,kubernetes/autoscaler,kubernetes/autoscaler,kgrygiel/autoscaler,kgrygiel/autoscaler,mmerrill3/autoscaler,mmerrill3/autoscaler,bskiba/autoscaler,kubernetes/autoscaler