Add user-facing documentation for the new CQL per-row TTL feature,
in docs/cql/cql-extensions.md.
Also mention (and link) the new alternative TTL feature in a few
relevant documents about the old (per-write) TTL, about CDC,
and about the CREATE TABLE and ALTER TABLE commands.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
34c0c64d9d