Updated comment on new external method

tsk/img/img_open.c

@@ -373,7 +373,9 @@ tsk_img_open_utf8(int num_img,
 /**
 * \ingroup imglib
  * Opens an an image of type TSK_IMG_TYPE_EXTERNAL. The void pointer parameter
- * must be castable to a TSK_IMG_INFO pointer.
+ * must be castable to a TSK_IMG_INFO pointer.  It is up to 
+ * the caller to set the tag value in ext_img_info.  This 
+ * method will initialize the cache lock. 
  *
  * @param ext_img_info Pointer to the partially initialized disk image
  * structure, having a TSK_IMG_INFO as its first member