[doc] Try to fix generating hb-features docs

Move it to a separate section, since hb-common says include hb.h, while
we want to include hb-features.h here.

This still does not fix generating documentation of undefined macros
(e.g. HB_HAS_GDI since we build docs on Linux).

docs/harfbuzz-docs.xml

@@ -56,6 +56,7 @@
         <xi:include href="xml/hb-blob.xml"/>
         <xi:include href="xml/hb-buffer.xml"/>
         <xi:include href="xml/hb-common.xml"/>
+        <xi:include href="xml/hb-features.xml"/>
         <xi:include href="xml/hb-draw.xml"/>
         <xi:include href="xml/hb-paint.xml"/>
         <xi:include href="xml/hb-deprecated.xml"/>

docs/harfbuzz-sections.txt

@@ -166,16 +166,6 @@ HB_TAG_MAX_SIGNED
 HB_LANGUAGE_INVALID
 HB_FEATURE_GLOBAL_END
 HB_FEATURE_GLOBAL_START
-HB_HAS_CAIRO
-HB_HAS_CORETEXT
-HB_HAS_DIRECTWRITE
-HB_HAS_FREETYPE
-HB_HAS_GDI
-HB_HAS_GLIB
-HB_HAS_GOBJECT
-HB_HAS_GRAPHITE
-HB_HAS_ICU
-HB_HAS_UNISCRIBE
 <SUBSECTION Private>
 HB_BEGIN_DECLS
 HB_END_DECLS
@@ -194,6 +184,20 @@ HB_DEPRECATED
 HB_DEPRECATED_FOR
 </SECTION>
 
+<SECTION>
+<FILE>hb-features</FILE>
+HB_HAS_CAIRO
+HB_HAS_CORETEXT
+HB_HAS_DIRECTWRITE
+HB_HAS_FREETYPE
+HB_HAS_GDI
+HB_HAS_GLIB
+HB_HAS_GOBJECT
+HB_HAS_GRAPHITE
+HB_HAS_ICU
+HB_HAS_UNISCRIBE
+</SECTION>
+
 <SECTION>
 <FILE>hb-draw</FILE>
 hb_draw_funcs_create

src/hb-features.h.in

@@ -27,6 +27,14 @@
 
 HB_BEGIN_DECLS
 
+/**
+ * SECTION: hb-features
+ * @title: hb-features
+ * @short_description: Feature detection
+ * @include: hb-features.h
+ *
+ * Macros for detecting optional HarfBuzz features at build time.
+ **/
 
 /**
  * HB_HAS_CAIRO: