oggopus: Address Stephen Farrell's IESG comments.

- Clarify that 125,829,120 is just 120 MB.
- Add a figure to Section 3 of an example logical stream.
- Add a reference for Q notation.
- Refer to the downmixing figures in the text.
- Clarify that user comments are UTF-8.
- Clarify that the -573 and 111 gain values are examples.
- Add specific advice to implementors on areas that have security
   implications.