Update doc comments.

The enumeration values cited here were out of date.  Rewrite to eliminate
the explicit mention of the enumeration value, to prevent it from
drifting out of date again in the future.

gtypelib.h

@@ -492,7 +492,6 @@ typedef struct {
  * @tag: A #GITypeTag
  * @interface: Index of the directory entry for the interface.
  * 
- * Types which are described by an entry in the typelib have a tag value of 21. 
  * If the interface is an enum of flags type, is_pointer is 0, otherwise it is 1.
  */
 typedef struct {
@@ -516,8 +515,7 @@ typedef struct {
  * direction as this one. 
  * @type: The type of the array elements.
  * 
- * Arrays have a tag value of 20. They are passed by reference, thus is_pointer 
- * is always 1.
+ * Arrays are passed by reference, thus is_pointer is always 1.
  */
 typedef struct {
   guint16 pointer         :1;