Documentation tweaks

- Use better grammar (it's -> its)
- Avoid repetition in some places
- Fix and shortify the docker install copy-paste snippet
Abuse shell history expansion to shortify docker snippet

GitHub wraps it with a horizontal scroller and that is annoying me.
Remove the explicit Rails3 hacks

Those will go into the web-console-rails3 project.
Maintain the Rails3 hacks in this project