kcms/rules: show explanatory text with contextual help button, not tooltip

Tooltips don't work when using the system with touch, and these here are
also a bit annoying with a pointing device since they appear instantly on
hover and take up a lot of space, so they unexpectedly pop up and cover
UI elements a lot.

To improve this, use a KCM.ContextualHelpButton inline in the list items
to show the explanation when clicked or hovered, just like we do in most
other places in other KCMs.

To make this work, the internal line breaks in the explanatory text had
to be removed to make the text flow properly with an arbitrarily-sized
tooltip.