docs: clarify the operator design doc a bit

[obnoxxx]

I the operator design document, custom resource definition(CRD) seems to have been used as a synonym for custom resource(CR) which is usually not correct.

This change makes the wording a little clearer by using CRD only when technically correct and using CR when appropriate.

Describe what this PR does

Make some wording in the operator design doc a little clearer and more correct.

Is there anything that requires special attention

no

Do you have any questions?

no

Is the change backward compatible?

yes

Are there concerns around backward compatibility?

no

Provide any external context for the change, if any.

none

Related issues

none

Future concerns

none

Checklist:

• [x] Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• [x] Reviewed the developer guide on Submitting a Pull Request
• [x] Pending release notes updated with breaking and/or notable changes for the next major release.
• [x] Documentation has been updated, if necessary.
• [x] Unit tests have been added, if necessary.
• [x] Integration tests have been added, if necessary.