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

[Juri Linkov]

>> > What should be possible is to find the customization by browsing the
>> > tab-bar Custom group.  If that is not currently possible (I didn't
>> > check), then we should probably do something to fix that.
>>
>> It isn't, FWIW.  Thanks.
>
> Juri, can we please fix this?  Thanks.

Please specify to what extent it should be customizable.

Currently configuration of these buttons is possible with e.g.

  (setq tab-bar-close-button
        (propertize " x"
                    'display
                    `(image :type svg
                            :file "checkbox-mixed.svg"
                            :margin ,tab-bar-button-margin
                            :ascent center)
                    'close-tab t
                    :help "Click to close tab"))

  (setq tab-bar-new-button
        (propertize " + "
                    'display
                    `(image :type svg
                            :file "checked.svg"
                            :margin ,tab-bar-button-margin
                            :ascent center)
                    :help "Click to add tab"))

that uses SVG images.  Also it's possible to use Unicode chars with

  (setq tab-bar-close-button (propertize " x" 'display (propertize "✕" 'face 
'tab-bar-tab-inactive) 'close-tab t))
  (setq tab-bar-new-button (propertize " + " 'display (propertize "➕" 'face 
'tab-bar-tab)))

So I'd simply add such examples to the docstrings of these variables
for emacs-28.