Documentation/op-guide: highlight defragment operation

[gyuho]

Clarify on #9222.

/cc @yudai

[codecov-io]

Codecov Report

Merging #9245 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #9245      +/-   ##
=========================================
- Coverage   75.86%   75.8%   -0.07%     
=========================================
  Files         363     363              
  Lines       30194   30194              
=========================================
- Hits        22907   22888      -19     
- Misses       5681    5702      +21     
+ Partials     1606    1604       -2

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
pkg/fileutil/purge.go	73.68% <0%> (-7.9%)	⬇️
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
etcdserver/api/v3rpc/lease.go	81.08% <0%> (-5.41%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
proxy/grpcproxy/watch.go	90.06% <0%> (-3.11%)	⬇️
pkg/adt/interval_tree.go	85.58% <0%> (-2.41%)	⬇️
integration/bridge.go	90.07% <0%> (-2.3%)	⬇️
clientv3/leasing/cache.go	87.77% <0%> (-1.12%)	⬇️
internal/mvcc/watchable_store.go	85.23% <0%> (-1.11%)	⬇️
... and 28 more

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[xiang90]

lgtm.