Documentation: add How to avoid botching up ioctls

I pointed some folks at this and they wondered why it wasn't in the
kernel Documentation directory. So now it is.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>