Update docs for startup page of configuration

Summary: After several resurfaces of GUI the current docs are not up-to-date.

Test Plan: I used an impression from D11382 to write this. Tested with checkXML5. An updated screenshot is needed.

Reviewers: #dolphin, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: ngraham, kfm-devel, kde-doc-english

Tags: #dolphin, #documentation

Differential Revision: https://phabricator.kde.org/D29220