Update docstring of v1beta1.PipelineObject

[JeromeJu]

Changes

This commit clarifies the v1beta1.PipelineObject by deleting the ClusterPipeline which had not been implemented.

/kind documentation

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[JeromeJu]

Also could we check if v1beta1.PipelineObject interface is used anywhere else that should not be replaced by just v1beta1.Pipeline since we are not having ClusterPipeline?
cc @vdemeester

[lbernick]

can we just delete pipelineobject?
Never mind-- I didn't realize you also have #6469, in that case this is fine

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[Yongxuanzhang]

/lgtm

[JeromeJu]

can we just delete pipelineobject?

Actually I also felt the same 🤔 but not sure if it is safe since that's an interface under apis?