mm, doc: update read_ahead_kb for MADV_HUGEPAGE

MADV_HUGEPAGE is a new addition to readahead with behavior distinct from
normal pages.  To prevent confusion, we should update the documentation
accordingly.

Link: https://lkml.kernel.org/r/20241113150711.1685-1-laoar.shao@gmail.com


Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>