Small misc text changes and important change to controller doc 

Can anybody check my change to the "### Setting Content Type" section?

Also, I could not replicate said code in a test project. I don't know if things changed recently.
Well, I haven't used phoenix in a months, and I was playing with it today reading this guide on the new website.

When I tried to replicated that functionality on a test project, I noticed that there was no need to name the format 'text' and then use a template called 'txt' (without the 'e'). So I removed that.

And, if you specify a template as a string and then specify a format for which you don't have a template, it will fallback to said template. That was perplexing for me. I was expecting an error. But I understand your point.