Migrate ZFS-* message descriptions (#49)

Prerequisite for changing the URLs in the command outputs from
zfsonlinux.org to openzfs.github.io/openzfs-docs/

Signed-off-by: Ryan Moeller <ryan@iXsystems.com>

docs/msg/ZFS-8000-6X/index.rst

@@ -0,0 +1,80 @@
+..
+   CDDL HEADER START
+
+   The contents of this file are subject to the terms of the
+   Common Development and Distribution License (the "License").
+   You may not use this file except in compliance with the License.
+
+   You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+   or http://www.opensolaris.org/os/licensing.
+   See the License for the specific language governing permissions
+   and limitations under the License.
+
+   When distributing Covered Code, include this CDDL HEADER in each
+   file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+   If applicable, add the following below this CDDL HEADER, with the
+   fields enclosed by brackets "[]" replaced with your own identifying
+   information: Portions Copyright [yyyy] [name of copyright owner]
+
+   CDDL HEADER END
+
+   Portions Copyright 2007 Sun Microsystems, Inc.
+
+.. highlight:: none
+
+Message ID: ZFS-8000-6X
+=======================
+
+Missing top level device
+------------------------
+
++-------------------------+--------------------------------------------+
+| **Type:**               | Error                                      |
++-------------------------+--------------------------------------------+
+| **Severity:**           | Critical                                   |
++-------------------------+--------------------------------------------+
+| **Description:**        | One or more top level devices are missing. |
++-------------------------+--------------------------------------------+
+| **Automated Response:** | No automated response will be taken.       |
++-------------------------+--------------------------------------------+
+| **Impact:**             | The pool cannot be imported.               |
++-------------------------+--------------------------------------------+
+
+.. rubric:: Suggested Action for System Administrator
+
+Run ``zpool import`` to list which pool cannot be imported:
+
+::
+
+   # zpool import
+     pool: test
+       id: 13783646421373024673
+    state: FAULTED
+   status: One or more devices are missing from the system.
+   action: The pool cannot be imported.  Attach the missing devices and try again.
+      see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-6X
+   config:
+
+           test              FAULTED   missing device
+             c0t0d0          ONLINE
+
+   Additional devices are known to be part of this pool, though their
+   exact configuration cannot be determined.
+
+ZFS attempts to store enough configuration data on the devices such
+that the configuration is recoverable from any subset of devices.  In
+some cases, particularly when an entire toplevel virtual device is
+not attached to the system, ZFS will be unable to determine the
+complete configuration.  It will always detect that these devices are
+missing, even if it cannot identify all of the devices.
+
+The pool cannot be imported until the unknown missing device is
+attached to the system. If the device has been made available in an
+alternate location, use the ``-d`` option to ``zpool import`` to search
+for devices in a different directory.  If the missing device is
+unavailable, then the pool cannot be imported.
+
+.. rubric:: Details
+
+The Message ID: ``ZFS-8000-6X`` indicates one or more top level
+devices are missing from the configuration.