memblock: fix kernel-doc for MEMBLOCK_RSRV_NOINIT

The kernel-doc description of MEMBLOCK_RSRV_NOINIT and
memblock_reserved_mark_noinit() do not accurately describe their
functionality.

Expand their kernel doc to make it clear that the user of
MEMBLOCK_RSRV_NOINIT is responsible to properly initialize the struct pages
for such regions and add more details about effects of using this flag.

Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/f8140a17-c4ec-489b-b314-d45abe48bf36@redhat.com
Link: https://lore.kernel.org/r/20250826071947.1949725-1-rppt@kernel.org


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>