BIO: Emphasise API contract for BIO_sendmmsg/BIO_recvmmsg

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #21905)

doc/man3/BIO_sendmmsg.pod

@@ -197,6 +197,10 @@ transient error, many of which are system specific.
 
 =back
 
+Third parties implementing custom BIOs supporting the BIO_sendmmsg() or
+BIO_recvmmsg() methods should note that it is a required part of the API
+contract that an error is always raised when either of these functions return 0.
+
 BIO_dgram_set_local_addr_enable() returns 1 if local address support was
 successfully enabled or disabled and 0 otherwise.