Documentation: add TLS offload documentation

Describe existing kernel TLS offload (added back in Linux 4.19) -
the mechanism, the expected behavior and the notable corner cases.

This documentation is mostly targeting hardware vendors who want
to implement offload, to ensure consistency between implementations.

v2:
- add emphasis around TLS_SW/TLS_HW/TLS_HW_RECORD;
- remove mentions of ongoing work (Boris);
- split the flow of data in SW vs. HW cases in TX overview
(Boris);
- call out which fields are updated by the device and which
are filled by the stack (Boris);
- move error handling into it's own section (Boris);
- add more words about fallback (Boris);
- note that checksum validation is required (Alexei);
- note that drivers shouldn't pay attention to the TLS
device features.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Dave Watson <davejwatson@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>