datadog-agent: Add release note entry.

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

···
           file</link> format.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The <literal>datadog-agent</literal>,
+          <literal>datadog-integrations-core</literal> and
+          <literal>datadog-process-agent</literal> packages were
+          upgraded from 6.11.2 to 7.30.2, git-2018-09-18 to 7.30.1 and
+          6.11.1 to 7.30.2, respectively. For details, see the
+          <link xlink:href="https://github.com/DataDog/datadog-agent/blob/main/CHANGELOG.rst">upstream
+          changelog</link>.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-21.11-notable-changes">

nixos/doc/manual/release-notes/rl-2111.section.md

···
 
 - The `todoman` package was upgraded from 3.9.0 to 4.0.0. This introduces breaking changes in the [configuration file](https://todoman.readthedocs.io/en/stable/configure.html#configuration-file) format.
 
+- The `datadog-agent`, `datadog-integrations-core` and `datadog-process-agent` packages were upgraded from 6.11.2 to 7.30.2, git-2018-09-18 to 7.30.1 and 6.11.1 to 7.30.2, respectively. For details, see the [upstream changelog](https://github.com/DataDog/datadog-agent/blob/main/CHANGELOG.rst).
+
 ## Other Notable Changes {#sec-release-21.11-notable-changes}
 
 - The setting [`services.openssh.logLevel`](options.html#opt-services.openssh.logLevel) `"VERBOSE"` `"INFO"`. This brings NixOS in line with upstream and other Linux distributions, and reduces log spam on servers due to bruteforcing botnets.