docs: Add information about ipc sysctls limitations

After 25b21cb ("[PATCH] IPC namespace core") and 4e98231
("[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>

Author: legionus

Documentation/admin-guide/sysctl/kernel.rst

@@ -541,6 +541,9 @@ default (``MSGMNB``).
 ``msgmni`` is the maximum number of IPC queues. 32000 by default
 (``MSGMNI``).
 
+All of these parameters are set per ipc namespace. The maximum number of bytes
+in POSIX message queues is limited by ``RLIMIT_MSGQUEUE``. This limit is
+respected hierarchically in the each user namespace.
 
 msg_next_id, sem_next_id, and shm_next_id (System V IPC)
 ========================================================
@@ -1181,15 +1184,20 @@ are doing anyway :)
 shmall
 ======
 
-This parameter sets the total amount of shared memory pages that
-can be used system wide. Hence, ``shmall`` should always be at least
-``ceil(shmmax/PAGE_SIZE)``.
+This parameter sets the total amount of shared memory pages that can be used
+inside ipc namespace. The shared memory pages counting occurs for each ipc
+namespace separately and is not inherited. Hence, ``shmall`` should always be at
+least ``ceil(shmmax/PAGE_SIZE)``.
 
 If you are not sure what the default ``PAGE_SIZE`` is on your Linux
 system, you can run the following command::
 
 	# getconf PAGE_SIZE
 
+To reduce or disable the ability to allocate shared memory, you must create a
+new ipc namespace, set this parameter to the required value and prohibit the
+creation of a new ipc namespace in the current user namespace or cgroups can
+be used.
 
 shmmax
 ======