cql3/expr: Clarify multi-column doesn't use indexing
cql3/expr: Clarify multi-column doesn't use indexing

Although not currently used, the old code was wrong and confusing to
readers.

Signed-off-by: Dejan Mircevski <1e9694aae17a51d4875d9bf4a0d0e20ef37c1b85@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla