doc: note that the BN_new() initialises the BIGNUM
BN_new() and BN_secure_new() not only allocate memory, but also
initialise it to deterministic value - 0.
Document that behaviour to make it explicit
@@ -36,7 +36,8 @@ If B<a> is NULL, nothing is done.
=head1 RETURN VALUES
BN_new() and BN_secure_new()
-return a pointer to the B<BIGNUM>. If the allocation fails,
+return a pointer to the B<BIGNUM> initialised to the value 0.
+If the allocation fails,
they return B<NULL> and set an error code that can be obtained
by L<ERR_get_error(3)>.
