Add DOCUMENTATION_URL as a standard value for /etc/os-release

It is very useful for distributions to be able to set a primary
documentation URL in a standard location so that users and
applications on the system can identify it. For example, many
headless systems these days use the "Cockpit" admin console. It
would be ideal if we could specify this location directly in the
os-release file so that any application or service could have a
well-known location for retrieving this and displaying it
appropriately. Users could likewise examine /etc/os-release to
learn this location.

Related: cockpit-project/cockpit#10198

Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>

man/os-release.xml

@@ -218,14 +218,18 @@
 
       <varlistentry>
         <term><varname>HOME_URL=</varname></term>
+        <term><varname>DOCUMENTATION_URL=</varname></term>
         <term><varname>SUPPORT_URL=</varname></term>
         <term><varname>BUG_REPORT_URL=</varname></term>
         <term><varname>PRIVACY_POLICY_URL=</varname></term>
 
         <listitem><para>Links to resources on the Internet related to 
-        the operating system. <varname>HOME_URL=</varname> should refer
-        to the homepage of the operating system, or alternatively some
-        homepage of the specific version of the operating system.
+        the operating system.
+        <varname>HOME_URL=</varname> should refer to the homepage of
+        the operating system, or alternatively some homepage of the
+        specific version of the operating system.
+        <varname>DOCUMENTATION_URL=</varname> should refer to the main
+        documentation page for this operating system.
         <varname>SUPPORT_URL=</varname> should refer to the main
         support page for the operating system, if there is any. This
         is primarily intended for operating systems which vendors