Improve doc strings in tab-line.el

Improve doc strings in tab-line.el

* lisp/tab-line.el (tab-line-tab-name-function)
(tab-line-tab-name-truncated-buffer, tab-line-tabs-mode-buffers)
(tab-line-tabs-buffer-group-function)
(tab-line-tabs-buffer-group-sort-function)
(tab-line-tabs-buffer-groups, tab-line-tab-name-format-function)
(tab-line-tab-name-format-default, tab-line-format-template)
(tab-line-tab-face-inactive-alternating)
(tab-line-tab-face-special, tab-line-tab-face-modified)
(tab-line-format, tab-line-auto-hscroll, tab-line-hscroll-right)
(tab-line-hscroll-left, tab-line-new-tab, tab-line-select-tab)
(tab-line-switch-to-prev-tab, tab-line-switch-to-next-tab)
(tab-line-close-tab-function, tab-line-close-tab)
(tab-line-tab-context-menu, tab-line-context-menu)
(tab-line-mode, tab-line-exclude-modes, tab-line-mode--turn-on):
Add or fix doc strings.