It's surprising how difficult it is to capture a screen shot with a "tooltip" in Windows. There is a rather simple method however.

If you have Office 365 installed, OneNote 2016 has a screen capture function, triggered by Windows-Shift-S. To get a screen show of a tooltip use the On Screen Keyboard (osk). First bring up OSK. Then press Windows, Shift on the OSK. Move your mouse over the thing with the tooltip, then press "s". It will trigger screen capture. You can then paste the image where you want to use it.

To save it into a PNG or JPEG, you'll need to paste into something like "mspaint" then save.