planner, executor: add some comments about how we decide sample rate (#34140)
planner, executor: add some comments about how we decide sample rate (#34140)

ref pingcap/tidb#24182
ngaut/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb