doc: announce mbuf layout change

doc: announce mbuf layout change

In order to prepare for adding more features requiring more space in mbuf,
some static fields must become dynamic.
Some small layout changes may have performance benefits as well.

The deprecation notice for atomic refcount is moved and reworded
to fit below the layout deprecation.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: David Marchand <david.marchand@redhat.com>