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:

* Use full options flags for clarity.
* Use `[branch-name]` consistently (brackets and `-name` suffix).
* Replace `<>` style code comments with sentence instructions.