Documentation/tcp: Add TCP-AO documentation

It has Frequently Asked Questions (FAQ) on RFC 5925 - I found it very
useful answering those before writing the actual code. It provides answers
to common questions that arise on a quick read of the RFC, as well as how
they were answered. There's also comparison to TCP-MD5 option,
evaluation of per-socket vs in-kernel-DB approaches and description of
uAPI provided.

Hopefully, it will be as useful for reviewing the code as it was for writing.

Cc: Jonathan Corbet <[email protected]>
Cc: [email protected]
Signed-off-by: Dmitry Safonov <[email protected]>
Acked-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Documentation/networking/index.rst

@@ -106,6 +106,7 @@ Contents:
    sysfs-tagging
    tc-actions-env-rules
    tc-queue-filters
+   tcp_ao
    tcp-thin
    team
    timestamping