bug#47117: 28.0.50; [PATCH] Update docstring of 'tab-line-tabs-function'

[Gabriel]

The user option 'tab-line-tabs-function' on tab-line.el does not mention
the possible value of 'tab-line-tabs-buffer-groups', which can difficult
the discoverability of this feature for users. This patch update its
docstring to mention the missing option, and adds a docstring to the
function of same name. Feel free to modify the proposed patch for an
improved and clearer docstring version.

0001-lisp-tab-line.el-Update-docstring-of-tab-line-tabs-f.patch
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