Update screenshots and cleanup text for xfce4-clipman-plugin

Submitted by Kevin Bowen @kevinbowen

Assigned to Kevin Bowen @kevinbowen

Link to original bug (#16614)

Description

The xfce4-clipman-plugin just 20200329) got a bigger release and updating the docs would be really meaningful right now.

This includes the screenshots, reformulating/reformatting the text (lots of bold-italic text and formatting overkill) and making sure all info is included (ochosi has added a brief section about the new feature yesterday night, but it could probably be improved).