Documentation/sysctl/kernel.txt: document /proc/sys/shmall

Signed-off-by: Carlos Alberto Lopez Perez <clopez@igalia.com>
Cc: Rob Landley <rob@landley.net>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>