iostats.txt: update it to cover recent Kernels

Everything there that it is said for 2.6 also applies on
current 4.x Kernels. So, update the information there.

While here, use ``foo`` for literals.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>