Added to EventContext.renderEach documentation

It took me quite a lot of experimentation and reading the sammy source code to understand how the different arguments worked, especially as the provided examples plainly ignore the `name` argument without explanation. I also explained that the data can come from the previous step in the chain (is that the right terminology for `previous_content`?), as it's used in the sample on the front page, but not explained.

I also fixed a typo.