Fixed #1105 -- Add hook for custom context in `BrowsableAPIRenderer`.

Replace hard coded response status check with `allow_form` context
variable, so that it can be overridden in a custom renderer class.
Refs #1109 -- Update docs. Integrate changes from feedback.
Merge pull request #1109 from thirstydigital/tickets/1105-renderer-context

Fixed #1105 -- Add hook for custom context in `BrowsableAPIRenderer`.