bug#68715: Undocumented base services in Guix manual

[Richard Sent <richard@freakingpenguin.com>]

Hi all,

Despite being exported by gnu/services/base.scm, and despite Section 11.9.1 Base
Services saying

--8<---------------cut here---------------start------------->8---
The ‘(gnu services base)’ module provides definitions for the basic
services that one expects from the system.  The services exported by
this module are listed below.
--8<---------------cut here---------------end--------------->8---

the following services and/or types are exported by base.scm, but are not
documented in the reference manual (commit 250477d869). I marked
extendable services with (e).

- fstab-service-type (e)
- root-file-system-service
- file-system-service-type (e)
- swap-service

An argument could be made for not documenting root-file-system-service
and swap-service, since they're a) not extensible b) not service
types and c) likely not relevant for the vast majority of users.
Personally I'd still lean towards documenting them, if only to match
what the manual says.

fstab-service-type and file-system-service-type, however, are extensible
and conceivably could be extended by a user. For instance, a custom NAS
client service that creates a mount point. As such, I believe those two
should be documented. That should help make them a bit more discoverable
to users.

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.