docs: port probing reference in its own chapter

docs/reference/api/ModemManager-overview.xml

@@ -70,9 +70,10 @@
         connected, not about the RS232 modem connected to the adapter, if any.
       </para>
     </section>
+  </chapter>
 
-    <section>
-      <title>Probing</title>
+  <chapter id="ref-overview-modem-port-probing">
+    <title>Port probing</title>
     <para>
       Whenever a new device is detected by ModemManager, port probing process will
       get started, so that we can determine which kind of ports we have, and also
@@ -454,10 +455,10 @@ mm_plugin_create (void)
         </programlisting>
       </example>
     </section>
-    </section>
+  </chapter>
 
-    <section>
-      <title>Port grabbing and Modem object creation</title>
+  <chapter id="ref-overview-modem-object-creation">
+    <title>Modem object creation</title>
     <para>
       Once a port passes all probing filters of a given plugin, the plugin will grab
       the port. When the first port of a given device is grabbed, the plugin will create
@@ -468,8 +469,6 @@ mm_plugin_create (void)
       port type hints can be used to specify AT port flags (e.g. if a port is to be
       considered primary, secondary or for PPP).
     </para>
-    </section>
-
   </chapter>
 
   <chapter id="ref-overview-modem-state-machine">