doc: announce changes in structure alignments for UBSan

As part of the effort to limit undefined behaviors in DPDK code,
some structure alignments revealed not to be accurate enough.
At the moment two structures have been identified but others may be
found while running more parts of DPDK with UBSan.

Though the calling code can explicitly request alignment,
we would prefer to put the alignment constraints as part of the
structure definition itself.

Such changes are ABI breakages.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>

Author: david-marchand

doc/guides/rel_notes/deprecation.rst

@@ -57,6 +57,10 @@ Deprecation Notices
   ``__atomic_thread_fence`` must be used for patches that need to be merged in
   20.08 onwards. This change will not introduce any performance degradation.
 
+* lib: Multiple issues relating to unaligned accesses have been detected using the UBSan checker.
+  As part of resolving those issues, alignment in some structures will be updated in 25.11,
+  namely (but not exhaustively): ``struct rte_stack_lf_head`` and ``struct rte_mp_msg``.
+
 * lib: will fix extending some enum/define breaking the ABI. There are multiple
   samples in DPDK that enum/define terminated with a ``.*MAX.*`` value which is
   used by iterators, and arrays holding these values are sized with this