bug#51648: 28.0.60; `tab-bar-close-button' and `tab-bar-new-button' should be documented in the manual

[Juri Linkov]

> People generally want to customize the image, and perhaps the scale of
> that image.
>
> But I see no harm in letting people customize the whole string, if he
> wants to.

I can't imagine how Customization UI could support customization of
such strings with text properties and images on them.