Documentation: Update augmented rbtree documentation

Documentation: Update augmented rbtree documentation

Current documentation referred to the old method of handling augmented
trees.  Update documentation to correspond with the changes done in
commit b945d6b2554d ("rbtree: Undo augmented trees performance damage
and regression").

Cc: Pekka Enberg <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Andrew Morton <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>