Document tab completion behavior.

[wting]

This is a bit of an undiscoverable feature that only gets mentioned in an obscure changelog entry.

[shaungrady]

And here we thought it was a bug…

[nodakai]

I have been searching for information on the strange "bug" I encountered. I googled for keywords such as "autojump tab completion underscore" and various variations. And today I decided to go through all the potentially relevant Github issues, and finally found this. 🤦