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.
I'm thinking about this one. The issue, unless I'm completely wrong, is that if the format is JSON, but there is no 'whatever.json' template, the atom form will blow up. For somebody who is new, this could be a very perplexing error. By including the format extension in the template name in the render call, it becomes pretty explicit what the template name should be. Maybe the solution is to expand the section on using atoms for template names and explain this better?

Btw, if we make a change here, 'symbol' should become 'atom'.