mbuf: document guideline for new fields and flags

Since dynamic fields and flags were added in 19.11,
the idea was to use them for new features, not only PMD-specific.

The guideline is made more explicit in doxygen, in the mbuf guide,
and in the contribution design guidelines.

For more information about the original design, see the presentation
https://www.dpdk.org/wp-content/uploads/sites/35/2019/10/DynamicMbuf.pdf

This decision was discussed in the Technical Board:
http://mails.dpdk.org/archives/dev/2020-June/169667.html

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>