bug#3528: 23.0.94; feature request: Add mode-line tooltips for buffer position (% of)and (line, col)

> We now have mouseover tooltips for most mode-line info.
> Please add tooltips to explain the buffer position (12% of...) and the
> line and column indicators.

I wasn't clear. We have such tooltips, but they are not very helpful. `Size
indication mode' is useless text. Say instead what this field is - how to
understand the text, and what happens if you click `mouse-1' here.

> We might also add a tooltip for the time, mentioning the timezone
> info. or UTC delta.