mq_open(2): document sysctl limit EINVAL and ENFILE conditions

Document two missing error conditions for mq_open(2):

- EINVAL: returned when mq_maxmsg exceeds kern.mqueue.maxmsg or
  mq_msgsize exceeds kern.mqueue.maxmsgsize.
- ENFILE: add kern.mqueue.maxmq sysctl name to the existing entry.

PR:		243209
Reviewed by:	mhorne
MFC after:	1 week
Signed-off-by:	Kit Dallege <xaum.io@gmail.com>
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2098

lib/libsys/mq_open.2

@@ -35,7 +35,7 @@
 .\" the referee document.  The original Standard can be obtained online at
 .\"	http://www.opengroup.org/unix/online.html.
 .\"
-.Dd May 15, 2024
+.Dd April 2, 2026
 .Dt MQ_OPEN 2
 .Os
 .Sh NAME
@@ -282,6 +282,24 @@ and either
 or
 .Va mq_msgsize
 was less than or equal to zero.
+.It Bq Er EINVAL
+.Dv O_CREAT
+was specified in
+.Fa oflag ,
+the value of
+.Fa attr
+is not
+.Dv NULL ,
+and either
+.Va mq_maxmsg
+exceeds the
+.Va kern.mqueue.maxmsg
+sysctl limit,
+or
+.Va mq_msgsize
+exceeds the
+.Va kern.mqueue.maxmsgsize
+sysctl limit.
 .It Bq Er EMFILE
 Too many message queue descriptors or file descriptors are currently in use
 by this process.
@@ -295,6 +313,9 @@ is longer than
 .Brq Dv NAME_MAX .
 .It Bq Er ENFILE
 Too many message queues are currently open in the system.
+The system limit is controlled by the
+.Va kern.mqueue.maxmq
+sysctl.
 .It Bq Er ENOENT
 .Dv O_CREAT
 is not set and the named message queue does not exist.