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
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4438)

doc/man3/BN_new.pod

@@ -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)>.