preview-mode help: comment about -ddebug-layout
preview-doc: Review "Engrave (Preview)" page
preview-mode: Rename configuration variables

Make them more consistent.
Noticed the need for it when writing documentation
preview-doc: Configuring Debug Modes

Add description how to configure Debug Modes through
configuration variables.

Now I think this is straightforward enough so we don't
need to clutter the GUI with yet one more Preferences page.
Merge pull request #215 from uliska/preview-doc

Finish documenting Preview Mode