Add a description field to pipeline workspace declarations

[ghost]

Changes

A description field can help rationalize how a workspace will be used
across multiple tasks in a pipeline without forcing the user to read and
interpret the tasks themselves.

This PR adds an optional description field to pipeline workspace
declarations to help with this. I would personally like this support to help when writing
example pipelines for catalog tasks. e.g. https://github.com/tektoncd/catalog/pull/182/files#diff-eee548fe380dc59c94cc87563f0d454cR82

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• Includes tests (if functionality changed/added)
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Reviewer Notes

If API changes are included, additive changes must be approved by at least two OWNERS and backwards incompatible changes must be approved by more than 50% of the OWNERS, and they must first be added in a backwards compatible way.

Release Notes

You can now include a description as part of your pipeline's workspace declarations to help users figure out how their volumes will be used in the pipeline.

[dibyom]

/lgtm

[tekton-robot]

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