docs: tls: document the TLS_TX_ZEROCOPY_RO

Add missing documentation for the TLS_TX_ZEROCOPY_RO opt-in.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Link: https://lore.kernel.org/r/20220610180212.110590-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>