mm: add comments to do_mmap(), mmap_region() and vm_mmap()

mm: add comments to do_mmap(), mmap_region() and vm_mmap()

It isn't always entirely clear to users the difference between do_mmap(),
mmap_region() and vm_mmap(), so add comments to clarify what's going on in
each.

This is compounded by the fact that we actually allow callers external to
mm to invoke both do_mmap() and mmap_region() (!), the latter of which is
really strictly speaking an internal memory mapping implementation detail.

Link: https://lkml.kernel.org/r/20241212113152.28849-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>