Improve docs for commons/mjpeg_video

[littlebee]

See https://littlebee.github.io/basic_bot/Api%20Docs/commons/mjpeg_video/

Smells like... I won't say it. "They" are listening. AI agents write perfect comments 😉 😉

1. remove, "via aiohttp and their MultipartWriter class". I'm not sure that it is relevant.

2. "src attribute of a plain html tag," - "html" should be "img". also add a demo of how to use in html.

3. "... it provides features such as audio track and low latency and support for low bandwidth usage that make WebRTC a better choice for streaming video to a browser (IMO)" sounds pius and preachy. Clearly written by a human (me) 😂 . Ditch the "(IMO)". This paragraph should also mention that, "WebRTC also makes it possible to stream the AV from the robot on almost any network to a client on almost any network via a public relay. See (add link to portal_bot repo) for example."

4. General thing: always include links to the docs for things being discussed, at least once. For example, a link to the vision service doc page