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/[email protected]
Signed-off-by: Yafang Shao <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: laoar

Documentation/ABI/stable/sysfs-block

@@ -594,6 +594,9 @@ Description:
 		[RW] Maximum number of kilobytes to read-ahead for filesystems
 		on this block device.
 
+		For MADV_HUGEPAGE, the readahead size may exceed this setting
+		since its granularity is based on the hugepage size.
+
 
 What:		/sys/block/<disk>/queue/rotational
 Date:		January 2009