[libhandy] doc: Add the HdyHeaderGroup section

[Alexander Mikhaylenko <alexm src gnome org>]

commit 0ca0c07a6cb96997bd3ee2f068e7a2c78dd7e403
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon Jun 29 17:34:02 2020 +0200

    doc: Add the HdyHeaderGroup section
    
    Fixes https://gitlab.gnome.org/GNOME/libhandy/-/issues/54

 src/hdy-header-group.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)
---
diff --git a/src/hdy-header-group.c b/src/hdy-header-group.c
index 0b7d41d2..e8287fad 100644
--- a/src/hdy-header-group.c
+++ b/src/hdy-header-group.c
@@ -9,6 +9,44 @@
 
 #include "hdy-header-group.h"
 
+/**
+ * SECTION:hdy-header-group
+ * @short_description: An object handling composite title bars.
+ * @Title: HdyHeaderGroup
+ * @See_also: #GtkHeaderBar, #HdyHeaderBar, #HdyLeaflet
+ *
+ * The #HdyHeaderGroup object handles the header bars of a composite title bar.
+ * It splits the window decoration across the header bars, giving the left side
+ * of the decorations to the leftmost header bar, and the right side of the
+ * decorations to the rightmost header bar.
+ * See hdy_header_bar_set_decoration_layout().
+ *
+ * The #HdyHeaderGroup:decorate-all property can be used in conjunction with
+ * #HdyLeaflet:folded when the title bar is split across the pages of a
+ * #HdyLeaflet to automatically display the decorations on all the pages when
+ * the leaflet is folded.
+ *
+ * You can nest header groups, which is convenient when you nest leaflets too:
+ * |[
+ * <object class="HdyHeaderGroup" id="inner_header_group">
+ *   <property name="decorate-all" bind-source="inner_leaflet" bind-property="folded" 
bind-flags="sync-create"/>
+ *   <headerbars>
+ *     <headerbar name="inner_header_bar_1"/>
+ *     <headerbar name="inner_header_bar_2"/>
+ *   </headerbars>
+ * </object>
+ * <object class="HdyHeaderGroup" id="outer_header_group">
+ *   <property name="decorate-all" bind-source="outer_leaflet" bind-property="folded" 
bind-flags="sync-create"/>
+ *   <headerbars>
+ *     <headerbar name="inner_header_group"/>
+ *     <headerbar name="outer_header_bar"/>
+ *   </headerbars>
+ * </object>
+ * ]|
+ *
+ * Since: 0.0.4
+ */
+
 /**
  * HdyHeaderGroupChildType:
  * @HDY_HEADER_GROUP_CHILD_TYPE_HEADER_BAR: The child is a #HdyHeaderBar