libgimpwidgets: move docs from template files to inline comments

but leave the template file for gimpstock there because its contents
would be too much for gimpstock.h.

libgimpwidgets/gimpwidgets.c

@@ -41,6 +41,17 @@
 #include "libgimp/libgimp-intl.h"
 
 
+/**
+ * SECTION: gimpwidgets
+ * @title: GimpWidgets
+ * @short_description: A collection of convenient widget constructors,
+ *                     standard callbacks and helper functions.
+ *
+ * A collection of convenient widget constructors, standard callbacks
+ * and helper functions.
+ **/
+
+
 /**
  * gimp_radio_group_new:
  * @in_frame:    %TRUE if you want a #GtkFrame around the radio button group.