doc: Add description of shard token bucket algo
doc: Add description of shard token bucket algo

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

avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar