Improve Documentation for format in Renderer

[cancan101]

1. The docs show a confusing . as the prefix for the format. For example:

.format: '.html'

whereas the class has:

class TemplateHTMLRenderer(BaseRenderer):
...
    format = 'html'

1. The docs don't give a suggestion as to what should format be set to when the media_type = 'image/*' or */*.

[tomchristie]

1. Sounds like it needs a docs pull request, yup.
2. What change would you suggest here? Either you'll need a custom conneg class, in order to handle multiple possible format extenstions, or you could use something generic, eg .img