Add to the SELECT documentation (docs/cql/dml/select.rst) documentation
of the new ability to select WRITETIME() and TTL() of a single element
of map, set or UDT.
Also in the TTL documentation (docs/cql/time-to-live.rst), which already
had a section on "TTL for a collection", add a mention of the ability
to read a single element's TTL(), and an example.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>