mm: improve the description for dirty_background_ratio/dirty_ratio sysctl

Now dirty_background_ratio/dirty_ratio contains a percentage of total
avaiable memory, which contains free pages and reclaimable pages.  The
number of these pages is not equal to the number of total system memory.
But they are described as a percentage of total system memory in
Documentation/sysctl/vm.txt.  So we need to fix them to avoid
misunderstanding.
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/vm.txt

@@ -119,8 +119,11 @@ other appears as 0 when read.
 
 dirty_background_ratio
 
-Contains, as a percentage of total system memory, the number of pages at which
-the background kernel flusher threads will start writing out dirty data.
+Contains, as a percentage of total available memory that contains free pages
+and reclaimable pages, the number of pages at which the background kernel
+flusher threads will start writing out dirty data.
+
+The total avaiable memory is not equal to total system memory.
 
 ==============================================================
 
@@ -151,9 +154,11 @@ interval will be written out next time a flusher thread wakes up.
 
 dirty_ratio
 
-Contains, as a percentage of total system memory, the number of pages at which
-a process which is generating disk writes will itself start writing out dirty
-data.
+Contains, as a percentage of total available memory that contains free pages
+and reclaimable pages, the number of pages at which a process which is
+generating disk writes will itself start writing out dirty data.
+
+The total avaiable memory is not equal to total system memory.
 
 ==============================================================