docs: tls: document the TLS_TX_ZEROCOPY_RO

Since it doesn't implement a full record layer, control

messages are not supported.

Optional optimizations

----------------------

There are certain condition-specific optimizations the TLS ULP can make,

if requested. Those optimizations are either not universally beneficial

or may impact correctness, hence they require an opt-in.

All options are set per-socket using setsockopt(), and their

state can be checked using getsockopt() and via socket diag (``ss``).

TLS_TX_ZEROCOPY_RO

~~~~~~~~~~~~~~~~~~

For device offload only. Allow sendfile() data to be transmitted directly

to the NIC without making an in-kernel copy. This allows true zero-copy

behavior when device offload is enabled.

The application must make sure that the data is not modified between being

submitted and transmission completing. In other words this is mostly

applicable if the data sent on a socket via sendfile() is read-only.

Modifying the data may result in different versions of the data being used

for the original TCP transmission and TCP retransmissions. To the receiver

this will look like TLS records had been tampered with and will result

in record authentication failures.

Statistics

==========