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/gimpcairo-utils.c

@@ -31,6 +31,16 @@
 #include "gimpcairo-utils.h"
 
 
+/**
+ * SECTION: gimpcairo-utils
+ * @title: GimpCairo-utils
+ * @short_description: Utility functions for cairo
+ *
+ * Utility functions that make cairo easier to use with common
+ * GIMP data types.
+ **/
+
+
 /**
  * gimp_cairo_set_source_rgb:
  * @cr:    Cairo context