Update rebasing/fetching in protocol guide

Functional changes:

* Don't forget to fetch before rebasing.
* Change Thin recommendation to Unicorn.
* Remove `--rebase` due to discussion
  [here](https://github.com/thoughtbot/dotfiles/pull/111).

Improvements for better readability/documentation:

* Replace `<>` style code comments with sentence instructions.
* Use full options flags (`--verbose`, `--delete`, etc.) for clarity.
* Use `<branch-name>` consistently (angle brackets and `-name` suffix).