oom: document obsolete oom_adj tunable

/proc/pid/oom_adj was deprecated in August 2010 with the introduction of
the new oom killer heuristic.

This patch copies the Documentation/feature-removal-schedule.txt entry for
this tunable to the Documentation/ABI/obsolete directory so nobody misses
it.
Signed-off-by: David Rientjes <rientjes@google.com>
Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/obsolete/proc-pid-oom_adj

+What:	/proc/<pid>/oom_adj
+When:	August 2012
+Why:	/proc/<pid>/oom_adj allows userspace to influence the oom killer's
+	badness heuristic used to determine which task to kill when the kernel
+	is out of memory.
+
+	The badness heuristic has since been rewritten since the introduction of
+	this tunable such that its meaning is deprecated.  The value was
+	implemented as a bitshift on a score generated by the badness()
+	function that did not have any precise units of measure.  With the
+	rewrite, the score is given as a proportion of available memory to the
+	task allocating pages, so using a bitshift which grows the score
+	exponentially is, thus, impossible to tune with fine granularity.
+
+	A much more powerful interface, /proc/<pid>/oom_score_adj, was
+	introduced with the oom killer rewrite that allows users to increase or
+	decrease the badness() score linearly.  This interface will replace
+	/proc/<pid>/oom_adj.
+
+	A warning will be emitted to the kernel log if an application uses this
+	deprecated interface.  After it is printed once, future warnings will be
+	suppressed until the kernel is rebooted.