Add note for printing all dbgmsg entries on FreeBSD

I looked for a bit, and couldn't find any documentation on
how to print all logged dbgmsg entries, just messages since
the DTrace probe started, until @allanjude kindly pointed me
toward the sysctl.

So let's add that note where the DTrace probe is mentioned for
FreeBSD, so other people can find it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12113

module/os/freebsd/zfs/zfs_debug.c

@@ -42,9 +42,12 @@ kstat_t *zfs_dbgmsg_kstat;
 /*
  * Internal ZFS debug messages are enabled by default.
  *
- * # Print debug messages
+ * # Print debug messages as they're logged
  * dtrace -n 'zfs-dbgmsg { print(stringof(arg0)); }'
  *
+ * # Print all logged dbgmsg entries
+ * sysctl kstat.zfs.misc.dbgmsg
+ *
  * # Disable the kernel debug message log.
  * sysctl vfs.zfs.dbgmsg_enable=0
  */