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>

Documentation/tty.txt

@@ -108,7 +108,9 @@ hardware driver through the function pointers within the tty->driver
 structure:
 
 write()			Write a block of characters to the tty device.
-			Returns the number of characters accepted.
+			Returns the number of characters accepted. The
+			character buffer passed to this method is already
+			in kernel space.
 
 put_char()		Queues a character for writing to the tty device.
 			If there is no room in the queue, the character is