mm: fadvise: document the fadvise(FADV_DONTNEED) behaviour for partial pages

A random seek IO benchmark appeared to regress because of a change to
readahead but the real problem was the benchmark.  To ensure the IO
request accesssed disk, it used fadvise(FADV_DONTNEED) on a block boundary
(512K) but the hint is ignored by the kernel.  This is correct but not
necessarily obvious behaviour.  As much as I dislike comment patches, the
explanation for this behaviour predates current git history.  Clarify why
it behaves like this in case someone "fixes" fadvise or readahead for the
wrong reasons.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>