Updates router migration guide to clarify caveat when using OnActivate
Updates router migration guide to clarify caveat when using OnActivate

PiperOrigin-RevId: 191502948

matanlurey/angular,angulardart/angular,chalin/angular2,chalin/angular2,matanlurey/angular,chalin/angular2,angulardart/angular,matanlurey/angular,angulardart/angular