doc: Document encrypted root partitions.

This is a followup to f7f292d3.

* doc/guix.texi (Preparing for Installation): Give commands for
encrypted root installation.
(Proceeding with the Installation): Add item about mapped devices.
(File Systems): Mention that 'dependencies' can list <mapped-device>
objects.
* gnu/system/examples/desktop.tmpl (mapped-devices): New field.
(file-systems): Add 'dependencies' field.