QUIC: Update documentation for SSL_get_[rw]poll_descriptor, SSL_net_(read|write)_desired

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21979)

doc/man3/SSL_get_rpoll_descriptor.pod

@@ -66,7 +66,14 @@ may change in response to any call to the SSL object other than
 SSL_net_read_desired(), SSL_net_write_desired(), SSL_get_rpoll_descriptor(),
 SSL_get_wpoll_descriptor() and SSL_get_event_timeout().
 
-These functions are not supported on non-QUIC SSL objects.
+On non-QUIC SSL objects, calls to SSL_get_rpoll_descriptor() and
+SSL_get_wpoll_descriptor() function the same as calls to
+BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor() on the respective read
+and write BIOs configured on the SSL object.
+
+On non-QUIC SSL objects, calls to SSL_net_read_desired() and
+SSL_net_write_desired() function identically to calls to SSL_want_read() and
+SSL_want_write() respectively.
 
 =head1 RETURN VALUES