This fixes regressions introduced in de1c158f ("system: Add support for
boot-time mapped devices.").

* doc/guix.texi (Initial RAM Disk): Adjust example with custom use of
  'base-initrd'.  Document #:mapped-devices parameter of 'base-initrd'.
* gnu/system/vm.scm (system-disk-image, system-qemu-image,
  virtualized-operating-system): Adjust call to 'base-initrd' to new
  signature.