add documentation about reclaim knob on proc.txt

This patch adds stuff about new reclaim field in proc.txt.

Change-Id: I1718fad12ec078e204e7a59769ce70fad7708756
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Patch-mainline: linux-mm @ 9 May 2013 16:21:29
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>