mm/page_alloc: clarify should_claim_block() commentary

mm/page_alloc: clarify should_claim_block() commentary

There's lots of text here but it's a little hard to follow, this is an
attempt to break it up and align its structure more closely with the code.

Reword the top-level function comment to just explain what question the
function answers from the point of view of the caller.

Break up the internal logic into different sections that can have their
own commentary describing why that part of the rationale is present.

Note the page_group_by_mobility_disabled logic is not explained in the
commentary, that is outside the scope of this patch...

Link: https://lkml.kernel.org/r/20250228-clarify-steal-v4-2-cb2ef1a4e610@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>