- update documentation

- use clear_bdi_congested/set_bdi_congested functions directly instead of
  old wrappers

- removed DECLARE_BUF_AS_STRING macro

Signed-off-by: Thomas Maier <balagi@justmail.de>
Cc: Peter Osterlund <petero2@telia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>