doc: Add Zoom tool description

To me, it was not obvious how to operate with Zoom tool. Iirc, I had a cursor glitch, that was shaped as a normal cursor, and nothing happened when I clicked on icon or on an image with selected tool.

I was looking for the explanation. The tooltip suggested "Pressed Shift for more". I pressed it, and it says "Zoom in and zoom out of the image". This explains nothing.

I then looked in the Handbook, and was surprised that documentation misses the Zoom tool.

I then found the status bar text "Zoom: Click to zoom in/out or left drag to zoom into a specific area.". This makes sense.

I also have discovered that double clicking on the tool resets zoom to the 1:1. This is also not documented anywhere.

This MR documents it.