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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/tty.txt

···
 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