util/chunked_vector: more complete comment
util/chunked_vector: more complete comment

chunked_vector was headed by short comment which didn't really explain
why it exists and how and why it really differs from std::dequeue.
Moreover, it made the vague claim that it "limits" contiguous
allocations, which it really doesn't (at least not in the asymptotic
sense).

In this patch I wrote a much longer comment, which I hope will clearly
explain exactly what chunked_vector is, how it really differs in its
contiguous allocations from std::deque, and what it guarantees and
doesn't guarantee.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

Closes #10857

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