doc: update maintainting V8 guide

- Mention the now existing Node.js canary branch.
- Clearer steps for backporting commits.
- Mention that `update-v8` can be used to simplify backports and
updates.
- Remove section about proposal for floating patches: embedder string
has been implemented.
- Wrap lines at 80 characters.

PR-URL: https://github.com/nodejs/node/pull/17260


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>