docs: Add system.clients description
docs: Add system.clients description

There's a document that sums up the tables from system keyspace and
its missing the clients table. This set is going to reimplement the
table keeping the schema intact, so it's good time to document it
right at the beginning.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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