document warning suppression

[jesse99]

There have been a lot of changes lately that result in many many warnings in user code. Many of these can be suppressed with the warn attribute, but there is no explanation for how to do this. I understand that the docs are lagging because it's more important to work on the language itself and it is a bit silly to document features in flux, but this is something that needs at least some minimal documentation in the tutorial or whatever.