xsk: Document tx_metadata_len layout

- how to use
- how to query features
- pointers to the examples

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231127190319.1190813-7-sdf@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>