Documentation: add TTY chapter

We now have all the kernel-doc comments in the code ready. So add a
couple of documents dragging those into generated docs from
Documentation/. There is only some sugar text around included
kernel-docs here.

It's a complete chapter, to be extended later as desired. This is a
solid cornerstone for the time being, I believe.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-24-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>