mempolicy: clarify what zone reclaim means

The zone_reclaim_mode API controls the reclaim behavior when a node runs
out of memory.  Contrary to its user-facing name, it is internally
referred to as "node_reclaim_mode".

This can be confusing.  But because we cannot change the name of the API
since it has been in place since at least 2.6, let's try to be more
explicit about what the behavior of this API is.

Change the description to clarify what zone reclaim entails, and be
explicit about the RECLAIM_ZONE bit, whose purpose has led to some
confusion in the past already [1] [2].

While at it, also soften the warning about changing these bits.

[joshua.hahnjy@gmail.com: remove the reference to the vm.zone_reclaim_mode sysctl as an ABI]
  Link: https://lkml.kernel.org/r/20250806134404.2000234-1-joshua.hahnjy@gmail.com
Link: https://lkml.kernel.org/r/20250805205048.1518453-1-joshua.hahnjy@gmail.com
Link: https://lore.kernel.org/linux-mm/1579005573-58923-1-git-send-email-alex.shi@linux.alibaba.com/ [1]
Link: https://lore.kernel.org/linux-mm/20200626003459.D8E015CA@viggo.jf.intel.com/ [2]
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: SeongJae Park <sj@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Byungchul Park <byungchul@sk.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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