Document Mode - an overview.

In response to http://lists.opensuse.org/yast-devel/2014-07/msg00039.html

This is just a summary of the existing state. Improvements welcome.

Before it gets to http://www.rubydoc.info/github/yast/yast-yast2/Yast/ModeClass , see
```bash
yardoc --use-cache  # -c
xdg-open doc/autodocs/Yast/ModeClass.html
```
maybe better use `@note` which is known yard tag.