services: shepherd: 'shepherd-service-type' requires documentation.

services: shepherd: 'shepherd-service-type' requires documentation.

* gnu/services/shepherd.scm (shepherd-service-type): Require a
'description' form.
* gnu/services/base.scm (root-file-system-service-type)
(rngd-service-type, host-name-service-type):
(virtual-terminal-service-type, console-keymap-service-type)
(syslog-service-type, swap-service-type)
(kmscon-service-type): Add description.
* gnu/services/networking.scm (dhcp-client-service-type): Likewise.
* gnu/system/install.scm (cow-store-service-type): Likewise.
* gnu/system/linux-container.scm (dummy-networking-service-type):
Likewise.
* gnu/system/mapped-devices.scm (device-mapping-service-type):
Likewise.
* tests/guix-system.sh: Likewise.