docs: document record duplication case where arg is NULL

docs/ares_dns_record.3

@@ -339,7 +339,7 @@ from \fIares_send_dnsrec(3)\fP or \fIares_search_dnsrec(3)\fP.  The structure
 to be duplicated is passed in the
 .IR dnsrec
 parameter, and the duplicated copy is returned, or NULL on error such as
-out of memory.
+out of memory, or if argument is NULL.
 
 The \fIares_dns_record_get_id(3)\fP function is used to retrieve the DNS
 message id from the DNS record provided in the

include/ares_dns_record.h

@@ -1103,6 +1103,8 @@ CARES_EXTERN ares_status_t ares_dns_write(const ares_dns_record_t *dnsrec,
 /*! Duplicate a complete DNS message.  This does not copy internal members
  *  (such as the ttl decrement capability).
  *
+ *  Returns NULL if \p dnsrec is NULL.
+ *
  *  \param[in] dnsrec Pointer to initialized and filled DNS record object.
  *  \return duplicted DNS record object, or NULL on out of memory.
  */