Document install status for Content Packs after upgrade

When upgrading to Graylog 3.0, all legacy Content Packs in the system will be still displayed in the Content Packs page. As there is no good way of knowing if those Content Packs were previously installed, we always show them as not installed, which is in my opinion the better solution.

To avoid any confusion when users upgrade their systems and see that a Content Pack they installed is somehow not indicating that, we should add a few sentences about this situation in the documentation.