Visual settings need to be documented

##### Orange version

3.34.0

##### Expected behavior
The documentation should contain explanations of the available visual settings for all widgets.

##### Actual behavior
There does not seem to be any documentation of the visual settings for the widgets – they are not always self-explanatory. E g, in the Line Plot widget there are settings for “Range” and “Selected range”. I thought these might adjust the boundaries for the Range in the Display settings, but apparently they represent the percentage of the grey scale value of the rendered Range. A pop-up would be fine.

##### Steps to reproduce the behavior
Go through the documentation pages at https://orangedatamining.com/widget-catalog/, notice the visual settings are not mentioned.
Open a widget and click the “Set visual settings” [a little redundant, eh?] button, hover over the settings, note no popups appear.

##### Additional info (worksheets, data, screenshots, ...)

Start by reviewing the widget-catalog pages and the Line Plot widget's “Set visual settings” controls, especially “Range” and “Selected range.” Document what the visual settings mean for the widgets, or add the suggested explanatory pop-ups; the widget documentation should no longer omit these settings.

Written by the indexing model from the issue text.