[PATCH] Update to Documentation/tty.txt on line disciplines

While trying to develop a line discipline I found a couple of things worth
mentioning in Documentation/tty.txt which weren't, so I decided to add
them.  It would be nice if someone more knowledgeable than me in that area
would look over them, in case I got something wrong.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>