mm/sparse: fix comment for section map alignment

mm/sparse: fix comment for section map alignment

The comment in mmzone.h currently details exhaustive per-architecture
bit-width lists and explains alignment using min(PAGE_SHIFT,
PFN_SECTION_SHIFT).  Such details risk falling out of date over time and
may inadvertently be left un-updated.

We always expect a single section to cover full pages.  Therefore, we can
safely assume that PFN_SECTION_SHIFT is large enough to accommodate
SECTION_MAP_LAST_BIT.  We use BUILD_BUG_ON() to ensure this.

Update the comment to accurately reflect this consensus, making it clear
that we rely on a single section covering full pages.

Link: https://lore.kernel.org/20260402102320.3617578-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Petr Tesarik <ptesarik@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>