docs: add cleanPolicy enum values

[george-e-shaw-iv]

• adds cleanPolicy enum values to the api reference documentation as described in the public website https://docs.pingcap.com/tidb-in-kubernetes/stable/backup-restore-cr

What problem does this PR solve?

Documentation does not state what the valid values of cleanPolicy is.

What is changed and how does it work?

Added documentation that lists and describes the valid values of cleanPolicy.

Code changes

• [ ] Has Go code change
• [ ] Has CI related scripts change

Tests

• [ ] Unit test
• [ ] E2E test
• [ ] Manual test
• [x] No code

Side effects

• [ ] Breaking backward compatibility
• [ ] Other side effects:

Related changes

• [ ] Need to cherry-pick to the release branch
• [ ] Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

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[ti-chi-bot[bot]]

Welcome @george-e-shaw-iv! It looks like this is your first PR to pingcap/tidb-operator 🎉

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 55.70%. Comparing base (a16b2e8) to head (38dd7d4). Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6064      +/-   ##
==========================================
- Coverage   57.19%   55.70%   -1.50%     
==========================================
  Files         259      266       +7     
  Lines       33233    46185   +12952     
==========================================
+ Hits        19008    25727    +6719     
- Misses      12291    18450    +6159     
- Partials     1934     2008      +74     

Flag	Coverage Δ
unittest	55.70% <ø> (-1.50%)	:arrow_down:

[csuzhangxc]

The api reference documentation is generated from the CRD Golang comments, you need to update the types.go and then run make generate to generate the new api reference documentation.

[george-e-shaw-iv]

I'm waiting on my company (Atlassian) to approve me signing the CLA and then I'll fix this PR and sign the SLA so it can be merged.