mm/page_alloc: explicitly define what alloc flags deplete min reserves

As there are more ALLOC_ flags that affect reserves, define what flags
affect reserves and clarify the effect of each flag.

Link: https://lkml.kernel.org/r/20230113111217.14134-5-mgorman@techsingularity.net


Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>