[docs] Add messing sections

Docs for these sections were not generated at all.

docs/harfbuzz-docs.xml

@@ -104,6 +104,8 @@
         <xi:include href="xml/hb-ot-font.xml"/>
         <xi:include href="xml/hb-ot-layout.xml"/>
         <xi:include href="xml/hb-ot-math.xml"/>
+        <xi:include href="xml/hb-ot-meta.xml"/>
+        <xi:include href="xml/hb-ot-metrics.xml"/>
         <xi:include href="xml/hb-ot-name.xml"/>
         <xi:include href="xml/hb-ot-shape.xml"/>
         <xi:include href="xml/hb-ot-var.xml"/>
@@ -123,6 +125,8 @@
         <xi:include href="xml/hb-graphite2.xml"/>
         <xi:include href="xml/hb-icu.xml"/>
         <xi:include href="xml/hb-uniscribe.xml"/>
+        <xi:include href="xml/hb-gdi.xml"/>
+        <xi:include href="xml/hb-directwrite.xml"/>
       </chapter>
 
       <!--chapter id="object-tree">

src/hb-directwrite.cc

@@ -33,6 +33,15 @@
 #include "hb-directwrite.h"
 
 
+/**
+ * SECTION:hb-directwrite
+ * @title: hb-directwrite
+ * @short_description: DirectWrite integration
+ * @include: hb-directwrite.h
+ *
+ * Functions for using HarfBuzz with DirectWrite fonts.
+ **/
+
 /* Declare object creator for dynamic support of DWRITE */
 typedef HRESULT (* WINAPI t_DWriteCreateFactory)(
   DWRITE_FACTORY_TYPE factoryType,

src/hb-gdi.cc

@@ -28,6 +28,16 @@
 
 #include "hb-gdi.h"
 
+
+/**
+ * SECTION:hb-gdi
+ * @title: hb-gdi
+ * @short_description: GDI integration
+ * @include: hb-gdi.h
+ *
+ * Functions for using HarfBuzz with GDI fonts.
+ **/
+
 static hb_blob_t *
 _hb_gdi_reference_table (hb_face_t *face HB_UNUSED, hb_tag_t tag, void *user_data)
 {

src/hb-ot-metrics.cc

@@ -33,6 +33,15 @@
 #include "hb-ot-face.hh"
 
 
+/**
+ * SECTION:hb-ot-metrics
+ * @title: hb-ot-metrics
+ * @short_description: OpenType Metrics
+ * @include: hb-ot.h
+ *
+ * Functions for fetching metrics from fonts.
+ **/
+
 static float
 _fix_ascender_descender (float value, hb_ot_metrics_tag_t metrics_tag)
 {