doc: Give an example with an encrypted root partition.

* gnu/system/examples/desktop.tmpl: Add 'mapped-devices' field.
  Use it in 'file-systems'.
* doc/guix.texi (System Installation): Suggest encrypted partitions.
  Give an example of a command sequence.

doc/guix.texi

@@ -5237,14 +5237,24 @@ Setting up network access is almost always a requirement because the
 image does not contain all the software and tools that may be needed.
 
 @item
-Unless this has already been done, you must partition and format the
-target partitions.
+Unless this has already been done, you must partition, optionally
+encrypt, and then format the target partitions.
 
 Preferably, assign partitions a label so that you can easily and
 reliably refer to them in @code{file-system} declarations (@pxref{File
 Systems}).  This is typically done using the @code{-L} option of
 @command{mkfs.ext4} and related commands.
 
+A typical command sequence may be:
+
+@example
+# fdisk /dev/sdX
+@dots{} Create partitions etc.@dots{}
+# cryptsetup luksFormat /dev/sdX1
+# cryptsetup open --type luks /dev/sdX1 my-partition
+# mkfs.ext4 -L my-root /dev/mapper/my-partition
+@end example
+
 The installation image includes Parted (@pxref{Overview,,, parted, GNU
 Parted User Manual}), @command{fdisk}, Cryptsetup/LUKS for disk
 encryption, and e2fsprogs, the suite of tools to manipulate

gnu/system/examples/desktop.tmpl

@@ -13,9 +13,17 @@
   ;; Assuming /dev/sdX is the target hard disk, and "root" is
   ;; the label of the target root file system.
   (bootloader (grub-configuration (device "/dev/sdX")))
+
+  ;; Here we assume that /dev/sdX1 contains a LUKS-encrypted
+  ;; root partition created with 'cryptsetup luksFormat'.
+  (mapped-devices (list (mapped-device
+                          (source "/dev/sdX1")
+                          (target "root-partition")
+                          (type luks-device-mapping))))
+
+  ;; Mount said encrypted partition.
   (file-systems (cons (file-system
-                        (device "root")
-                        (title 'label)
+                        (device "/dev/mapper/root-partition")
                         (mount-point "/")
                         (type "ext4"))
                       %base-file-systems))