Doc render

Frobbing the narrative text to make sure the reader understands the role dictionaries play in view callables/rendering.  I had a problem getting this and neither the docs on the 'starter' project nor the explanation of the rendering system helped much.

I remember getting this on first reading, but then when I went to implement I'd forgotten and just couldn't find what I needed in the text as written.

(If you're going to make the pyramid.reload_templates feature stand out in the docs, well, how to get values to renderers is a lot more important to get across.)