docs: Add documentation for mounting pxar archives via FUSE.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

docs/administration-guide.rst

@@ -429,6 +429,31 @@ files ending in ``.conf``.
 
 .. todo:: Explain interactive restore in more detail
 
+Mounting of Archives via FUSE
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The :term:`FUSE` implementation for the pxar archive allows you to mount a
+file archive as a read-only filesystem to a mountpoint on your host.
+
+.. code-block:: console
+
+  # proxmox-backup-client mount host/backup-client/2020-01-29T11:29:22Z root.pxar /mnt
+  # ls /mnt
+  bin   dev  home  lib32  libx32      media  opt   root  sbin  sys  usr
+  boot  etc  lib   lib64  lost+found  mnt    proc  run   srv   tmp  var
+
+This allows you to access the full content of the archive in a seamless manner.
+
+.. note:: As the FUSE connection needs to fetch and decrypt chunks from the
+    backup servers datastore, this can cause some additional network and CPU
+    load on your host, depending on the operations you perform on the mounted
+    filesystem.
+
+To unmount the filesystem simply use the ``umount`` command on the mountpoint:
+
+.. code-block:: console
+
+  # umount /mnt
 
 Login and Logout
 ~~~~~~~~~~~~~~~~

docs/epilog.rst

@@ -20,6 +20,7 @@
 .. _Virtual machine: https://en.wikipedia.org/wiki/Virtual_machine
 .. _APT: http://en.wikipedia.org/wiki/Advanced_Packaging_Tool
 .. _QEMU: https://www.qemu.org/
+.. _FUSE: https://en.wikipedia.org/wiki/Filesystem_in_Userspace
 
 .. _Client-server model: https://en.wikipedia.org/wiki/Client-server_model
 .. _AE: https://en.wikipedia.org/wiki/Authenticated_encryption

docs/glossary.rst

@@ -40,3 +40,11 @@ Glossary
 
       Is an easy-to-read, what-you-see-is-what-you-get plaintext
       markup syntax and parser system.
+
+   `FUSE`_
+
+      Filesystem in Userspace (FUSE) defines an interface which allows to
+      implement a filesystem in userspace as opposed to implementing it
+      in the kernel. The fuse kernel driver handles filesystem requests and
+      sends them to an userspace application for reply.
+