Add doc for `:touch` option of AR::Base#save

ActiveRecord::Base `save` and `save!` take an option boolean `:touch` parameter since #18225 (stems from #18202).

This commit documents that parameter.

![new](https://cloud.githubusercontent.com/assets/10076/5574294/713e80d0-8fbb-11e4-8c2b-a492fd0c8331.png)

[ci skip]