doc: make RTE_NEXT_ABI optional in guidelines
doc: make RTE_NEXT_ABI optional in guidelines

Initial process requires oncoming changes described in deprecation
notice should be implemented in a RTE_NEXT_ABI gated way.

This has been discussed in technical board, and since this can cause a
multiple #ifdef blocks in multiple locations of the code, can be
confusing specially for the modifications that requires data structure
changes. Anyway this was not happening in practice.

Making RTE_NEXT_ABI usage more optional based on techboard decision:
http://mails.dpdk.org/archives/dev/2019-January/123519.html

The intention with using RTE_NEXT_ABI was to provide more information
to the user about planned changes, and force developer to think more in
coding level. Since RTE_NEXT_ABI become optional, now the preferred way
to do this is, if possible, sending changes, described in deprecation
notice, as a separate patch and reference it in deprecation notice.

Signed-off-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>
Acked-by: Neil Horman <3316dc2d77df57653443c0391a5296176d6ca9c3@tuxdriver.com>
Acked-by: Kevin Traynor <1f46c93815774a2e602547cf1e37c682d62b566d@redhat.com>

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