mm: document improved handling of swappiness==0

Prior to commit fe35004f ("mm: avoid swapping out with
swappiness==0") setting swappiness to 0, reclaim code could still evict
recently used user anonymous memory to swap even though there is a
significant amount of RAM used for page cache.

The behaviour of setting swappiness to 0 has since changed.  When set,
the reclaim code does not initiate swap until the amount of free pages
and file-backed pages, is less than the high water mark in a zone.

Let's update the documentation to reflect this.

[akpm@linux-foundation.org: remove comma, per Randy]
Signed-off-by: NAaron Tomlin <atomlin@redhat.com>
Acked-by: NRik van Riel <riel@redhat.com>
Acked-by: NBryn M. Reeves <bmr@redhat.com>
Cc: Satoru Moriya <satoru.moriya@hds.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/vm.txt

@@ -696,7 +696,9 @@ swappiness
 
 This control is used to define how aggressive the kernel will swap
 memory pages.  Higher values will increase agressiveness, lower values
-decrease the amount of swap.
+decrease the amount of swap.  A value of 0 instructs the kernel not to
+initiate swap until the amount of free and file-backed pages is less
+than the high water mark in a zone.
 
 The default value is 60.