Add more note/highlight for scaling in storageD?

[wey-gu]

We may add more notes here: https://docs.nebula-graph.com.cn/2.0.1/8.service-tuning/load-balance/#storage_1 around:

• it will be removed in show hosts by default in 1 day when a stroageD was removed(in addition to it's in OFFLINE state)
• the timer is controlled by metaD config removed_threshold_sec

context:
https://nebulagraph.slack.com/archives/CSK0RSBFT/p1627912491003500

[wey-gu]

Can be closed via #1028
While maybe we could also add removed_threshold_sec into the metad conf chapter rather than only in the note part of host removal?

[cooper-lzy]

Only if the parameter exists in the configuration file, our document describes it.