mm/compaction: improve comment of is_via_compact_memory

mm/compaction: improve comment of is_via_compact_memory

We do proactive compaction with order == -1 via
1. /proc/sys/vm/compact_memory
2. /sys/devices/system/node/nodex/compact
3. /proc/sys/vm/compaction_proactiveness
Add missed situation in which order == -1.

Link: https://lkml.kernel.org/r/20230901155141.249860-6-shikemeng@huaweicloud.com
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>