Add example for update_all vs. update
Add example for update_all vs. update

This is unrelated to update! and thought this might be a good place
to talk about the class method but changed my mind.

Still I think this example is valuable. [ci skip]

rails/rails,kddeisz/rails,georgeclaghorn/rails,MSP-Greg/rails,Erol/rails,BlakeWilliams/rails,BlakeWilliams/rails,rails/rails,MSP-Greg/rails,Edouard-chin/rails,Vasfed/rails,georgeclaghorn/rails,Stellenticket/rails,MSP-Greg/rails,MSP-Greg/rails,Stellenticket/rails,Vasfed/rails,yahonda/rails,yawboakye/rails,Envek/rails,lcreid/rails,georgeclaghorn/rails,BlakeWilliams/rails,kmcphillips/rails,Erol/rails,BlakeWilliams/rails,Erol/rails,Envek/rails,georgeclaghorn/rails,Erol/rails,Envek/rails,esparta/rails,EmmaB/rails-1,kddeisz/rails,yawboakye/rails,Stellenticket/rails,lcreid/rails,Envek/rails,joonyou/rails,rails/rails,yawboakye/rails,Edouard-chin/rails,joonyou/rails,kmcphillips/rails,esparta/rails,yahonda/rails,kddeisz/rails,yawboakye/rails,yahonda/rails,kmcphillips/rails,Edouard-chin/rails,lcreid/rails,Stellenticket/rails,rails/rails,kmcphillips/rails,EmmaB/rails-1,esparta/rails,yahonda/rails,joonyou/rails,Vasfed/rails,kddeisz/rails,EmmaB/rails-1,joonyou/rails,esparta/rails,Vasfed/rails,Edouard-chin/rails,lcreid/rails