Upgrade Doc: assets:precompile:all was removed on 4

This is pretty small, but worth mentioning.

rails 4: https://github.com/rails/sprockets-rails/blob/a1b1e5ef632d9aeeaad71351956b1493e48a8643/lib/sprockets/rails/task.rb#L48

rails 3: https://github.com/rails/rails/blob/3-2-stable/actionpack/lib/sprockets/assets.rake#L59

review @rafaelfranca @guilleiguaran