docs: fix documentation for WpMetadataItem

docs/rst/library/c_api/metadata_api.rst

@@ -7,6 +7,7 @@ PipeWire Metadata
 
    digraph inheritance {
       rankdir=LR;
+      GBoxed -> WpMetadataItem
       GObject -> WpObject;
       WpObject -> WpProxy;
       WpProxy -> WpGlobalProxy;

lib/wp/metadata.c

@@ -319,6 +319,11 @@ wp_metadata_class_init (WpMetadataClass * klass)
       G_TYPE_UINT, G_TYPE_STRING, G_TYPE_STRING, G_TYPE_STRING);
 }
 
+/*!
+ * \struct WpMetadataItem
+ *
+ * WpMetadataItem holds the subject, key, type and value of a metadata entry.
+ */
 struct _WpMetadataItem
 {
   WpMetadata *metadata;