GNotification: finish documentation

Add the missing documentation and add the section to the GIO reference
docs.  Fix up a few small documentation issues.

https://bugzilla.gnome.org/show_bug.cgi?id=688492

docs/reference/gio/Makefile.am

@@ -59,6 +59,8 @@ IGNORE_HFILES = \
 	gnetworkingprivate.h		\
 	gnetworkmonitorbase.h		\
 	gnetworkmonitornetlink.h	\
+	gnotificationbackend.h		\
+	gnotification-private.h		\
 	gpollfilemonitor.h		\
 	gregistrysettingsbackend.h	\
 	gsettingsbackendinternal.h	\

docs/reference/gio/gio-docs.xml

@@ -223,6 +223,7 @@
         <xi:include href="xml/gmenu.xml"/>
         <xi:include href="xml/gmenuexporter.xml"/>
         <xi:include href="xml/gdbusmenumodel.xml"/>
+        <xi:include href="xml/gnotification.xml"/>
     </chapter>
     <chapter id="extending">
         <title>Extending GIO</title>

docs/reference/gio/gio-sections.txt

@@ -2980,6 +2980,9 @@ g_application_quit
 g_application_activate
 g_application_open
 <SUBSECTION>
+g_application_send_notification
+g_application_withdraw_notification
+<SUBSECTION>
 g_application_run
 g_application_set_default
 g_application_get_default
@@ -4150,3 +4153,29 @@ G_SUBPROCESS_LAUNCHER
 G_TYPE_SUBPROCESS_LAUNCHER
 g_subprocess_launcher_get_type
 </SECTION>
+
+<SECTION>
+<FILE>gnotification</FILE>
+<TITLE>GNotification</TITLE>
+GNotification
+g_notification_new
+<SUBSECTION>
+g_notification_set_title
+g_notification_set_body
+g_notification_set_icon
+g_notification_set_urgent
+<SUBSECTION>
+g_notification_set_default_action
+g_notification_set_default_action_and_target
+g_notification_set_default_action_and_target_value
+<SUBSECTION>
+g_notification_add_button
+g_notification_add_button_with_target
+g_notification_add_button_with_target_value
+<SUBSECTION Standard>
+G_IS_NOTIFICATION
+G_NOTIFICATION
+G_TYPE_NOTIFICATION
+G_TYPE_NOTIFICATION_BACKEND
+g_notification_get_type
+</SECTION>