bug#37829: 27.0.50; Overlay behaviour changed without documentation.

[Eli Zaretskii]

> Date: Mon, 21 Oct 2019 00:14:18 +0800
> From: Zhu Zihao <all_but_last@163.com>
> Cc: Zhu Zihao <all_but_last@163.com>,
>       37829@debbugs.gnu.org
> 
> 
> I'm puzzled that we already have :extend, but when I set the end of an overlay
> to the EOL, the face of overlay still don't cover the whole line.

The position you use as the end of the overlay is one position after
the end of the text that is affected by the overlay.  Remember: in
Emacs, a buffer position is between two characters.