docs: add usage restriction introduction of Pitr

[mlycore]

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[codecov-commenter]

Codecov Report

Merging #453 (10d0760) into main (3604a24) will decrease coverage by 3.40%.
Report is 188 commits behind head on main.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   39.71%   36.32%   -3.40%     
==========================================
  Files          51       59       +8     
  Lines        6083     6615     +532     
==========================================
- Hits         2416     2403      -13     
- Misses       3448     3985     +537     
- Partials      219      227       +8     

Files	Coverage Δ
pitr/cli/internal/cmd/root.go	86.00% <100.00%> (ø)
...re-operator/pkg/reconcile/computenode/container.go	70.96% <ø> (ø)
...e-operator/pkg/reconcile/computenode/deployment.go	80.47% <ø> (ø)
...here-operator/pkg/reconcile/computenode/service.go	0.00% <ø> (ø)
...sphere-operator/pkg/reconcile/computenode/utils.go	82.56% <ø> (ø)
...e-operator/pkg/reconcile/storagenode/aws/aurora.go	17.94% <ø> (-0.24%)	⬇️
...here-operator/pkg/reconcile/storagenode/aws/aws.go	0.00% <ø> (ø)
...erator/pkg/reconcile/storagenode/aws/rdscluster.go	0.00% <ø> (ø)
...gsphere-operator/pkg/kubernetes/service/builder.go	76.92% <0.00%> (ø)
pitr/agent/internal/handler/healthcheck.go	35.71% <33.33%> (ø)
... and 22 more

... and 2 files with indirect coverage changes

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