Improve the documentation of tab-bar and tab-line

Improve the documentation of tab-bar and tab-line

* doc/emacs/frames.texi (Tab Bars): Add xref to "Tab Line".
Document more commands.

* doc/emacs/windows.texi (Windows):
* doc/emacs/emacs.texi (Top): Add "Tab Line" menu.

* doc/emacs/windows.texi (Window Convenience):
Move tab-line documentation to new node "Tab Line".
(Tab Line): New node.

* doc/emacs/glossary.texi (Glossary):
* doc/emacs/modes.texi (Minor Modes):
* doc/emacs/display.texi (Standard Faces): Add xref to "Tab Line".