Document vendor override files

Features without documentation, tsk tsk.

docs/reference/gio/glib-compile-schemas.xml

@@ -35,6 +35,12 @@ specified in the <envar>XDG_DATA_DIRS</envar> environment variable. The
 usual location to install schema files is
 <filename>/usr/share/glib-2.0/schemas</filename>.
 </para>
+<para>
+In addition to schema files, glib-compile-schemas reads 'vendor override'
+files, which are key files that can override default values for keys in
+the schemas. The group names in the key files are the schema id, and the
+values are written in serialized GVariant form.
+</para>
 
 <refsect2><title>Options</title>
 <variablelist>

gio/gsettings.c

@@ -144,6 +144,26 @@
  * ]]></programlisting></example>
  *
  * <refsect2>
+ *   <title>Vendor overrides</title>
+ *   <para>
+ *     Default values are defined in the schemas that get installed by
+ *     an application. Sometimes, it is necessary for a vendor or distributor
+ *     to adjust these defaults. Since patching the XML source for the schema
+ *     is inconvenient and error-prone,
+ *     <link linkend="glib-compile-schemas">glib-compile-schemas</link> reads
+ *     so-called 'vendor override' files. These are keyfiles in the same
+ *     directory as the XML schema sources which can override default values.
+ *     The schema id serves as the group name in the key file, and the values
+ *     are expected in serialized GVariant form, as in the following example:
+ *     <informalexample><programlisting>
+ *     [org.gtk.Example]
+ *     key1='string'
+ *     key2=1.5
+ *     </programlisting></informalexample>
+ *   </para>
+ * </refsect2>
+ *
+ * <refsect2>
  *   <title>Binding</title>
  *   <para>
  *     A very convenient feature of GSettings lets you bind #GObject properties