[WIP] Add a Systemd library description

.yardopts

@@ -1 +1 @@
---no-private --markup markdown --protected library/*/src/**/*.rb --readme README.md --output-dir ./doc/autodocs - library/cwm/doc/CWM.md library/systemd/doc/services_and_sockets.md
+--no-private --markup markdown --protected library/*/src/**/*.rb --readme README.md --output-dir ./doc/autodocs - library/cwm/doc/CWM.md library/systemd/doc/services_and_sockets.md library/system/doc/system_services.md

library/system/doc/system_services.md

@@ -0,0 +1,31 @@
+# System Services Library
+
+The system services library offers an API to interact with system services, allowing the user to
+perform typical operations like querying the services, starting or stopping them, etc.
+
+The set of classes which are included in this library can be divided into:
+
+* A high level API which offers some abstractions on top of Systemd.
+* A low level one to talk closely to Systemd units (including services and sockets).
+
+Additionally, a widget that can be used in YaST modules (like yast2-dns-server) is included.
+
+## High Level API
+
+The high level API is composed by these classes:
+
+* {Yast2::SystemService}: represents a service (like `cups` or `dbus`) from a high level point of
+  view. Systemd concepts like *units* or *sockets* are abstracted by this class.
+* {Yast2::CompoundService}: groups a set of related services that might be handled together.
+  Think, for instance, about `iscsi`, `iscsid` and `iscsiuio` services in `yast2-iscsi-client`.
+  This class offers basically the same API than {Yast2::SystemService}.
+
+## Low Level API
+
+The low level API can be more convenient in some situations and it is basically composed of a set of
+classes that map to Systemd concepts: {Yast2::Systemd::Unit}, {Yast2::Systemd::Service},
+{Yast2::Systemd::Socket} and {Yast2::Systemd::Target}.
+
+## Service Widget
+
+Additionally to the classes to interact with system services, this library offers a widget...

library/systemd/src/lib/yast2/system_service.rb

@@ -32,7 +32,7 @@ module Yast2
   # See also {file:library/systemd/doc/services_and_sockets.md}.
   #
   # @note All changes performed over an object of this class are not applied into the
-  # underlying system until the {#save} method is called.
+  #   underlying system until the {#save} method is called.
   #
   # @example Enabling a service
   #   cups = SystemService.find("cups")