docs, tests and bug fix for pull 122

This is a continuation of the pull https://github.com/fabpot/Twig-extensions/pull/112

- [ ] `doc/i18n.rst` now includes the `notes` usage and example
- [ ] Added 3 test cases for different `notes` combinations
- [ ] As `notes` adds an inline php comment to the compiled template line breaks should be removed 
missing double-` around gettext.