*: add "--cluster" flag to defrag, highlight defrag endpoints

[gyuho]

Had a customer hit database size limit issue, but the doc was not clear about --endpoints flag in defrag command.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@55a7b30). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #9390   +/-   ##
=========================================
  Coverage          ?   72.54%           
=========================================
  Files             ?      362           
  Lines             ?    30783           
  Branches          ?        0           
=========================================
  Hits              ?    22330           
  Misses            ?     6826           
  Partials          ?     1627

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/defrag_command.go	37.2% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55a7b30...9919e4d. Read the comment docs.

[gyuho]

Merging. Low risk.