docs: Add information about ipc sysctls limitations

After 25b21cb2 ("[PATCH] IPC namespace core") and 4e982311
("[PATCH] IPC namespace - shm") the shared memory page count stopped
being global and started counting per ipc namespace. The documentation
and shmget(2) still says that shmall is a global option.

shmget(2):

SHMALL System-wide limit on the total amount of shared memory, measured
in units of the system page size. On Linux, this limit can be read and
modified via /proc/sys/kernel/shmall.

I think the changes made in 2006 should be documented.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lkml.kernel.org/r/ede20ddf7be48b93e8084c3be2e920841ee1a641.1663756794.git.legion@kernel.org


Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>