doc: pipelinerun GC description

[LinuxSuRen]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for reviewers:

Please check the following list before waiting reviewers:

• [ ] Already committed the CRD files to the Helm Chart if you created some new CRDs
• [ ] Already added the permission for the new API
• [ ] Already added the RBAC markers for the new controllers

Does this PR introduce a user-facing change??

None

[zheng1]

/lgtm /approve

[ks-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chilianyi, zheng1

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• ~~OWNERS~~ [chilianyi,zheng1]

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