man2/: Add MPOL_WEIGHTED_INTERLEAVE documentation

man2/: Add MPOL_WEIGHTED_INTERLEAVE documentation

Add documentation for the new MPOL_WEIGHTED_INTERLEAVE mode in the same
manual pages that mention MPOL_INTERLEAVE; namely, mbind(2),
set_mempolicy(2), and get_mempolicy(2).

Descriptions were based on the changes introduced in this patch:
<https://lore.kernel.org/all/20240202170238.90004-4-gregory.price@memverge.com/>

Which was upstreamed to 6.9 here:
<https://lore.kernel.org/linux-mm/20240313200532.34e4cff216acd3db8def4637@linux-foundation.org/>

Cc: gregory.price@memverge.com
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Svetly Todorov <svetly.todorov@memverge.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>