doc update

lib/element.c

diff --git a/lib/element.c b/lib/element.c
index 3ae7740..756e41a 100644
--- a/lib/element.c
+++ b/lib/element.c
@@ -753,7 +753,8 @@
  *   %ASN1_VALUE_NOT_FOUND if there isn't any value for the element
  *   selected, and %ASN1_MEM_ERROR if The value vector isn't big enough
  *   to store the result, and in this case @len will contain the number of
- *   bytes needed.
+ *   bytes needed. On the occasion that the stored data are of zero-length
+ *   this function may return %ASN1_SUCCESS even if the provided @len is zero.
  **/
 int
 asn1_read_value (asn1_node root, const char *name, void *ivalue, int *len)
@@ -826,7 +827,8 @@
  *   %ASN1_VALUE_NOT_FOUND if there isn't any value for the element
  *   selected, and %ASN1_MEM_ERROR if The value vector isn't big enough
  *   to store the result, and in this case @len will contain the number of
- *   bytes needed.
+ *   bytes needed. On the occasion that the stored data are of zero-length
+ *   this function may return %ASN1_SUCCESS even if the provided @len is zero.
  **/
 int
 asn1_read_value_type (asn1_node root, const char *name, void *ivalue,