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

[Po Lu]

I wanted to find out how to customize the image used for tab bar
buttons, but could not find anything in the manual.

It seems that setting the display property of `tab-bar-close-button' and
`tab-bar-new-button' is the way to go.

Shouldn't such an important customization be documented somewhere in the
manual?

Thanks.