tty: Clarify documentation of ->write()

The tty driver write method is different to the usual fops device write
methods as the buffer is already in kernel space. Clarify the docs since
someone writing a driver made that mistake.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>