docs(stores): clarify optionChoiceNames is write-only and variant ID mapping is positional

[hadarzaguri]

Summary

• Clarifies that optionChoiceNames is for requests only — the API always returns optionChoiceIds in responses, not optionChoiceNames
• Adds guidance that after a PATCH, variant IDs should be mapped to choices by position (response order matches request order), since optionChoiceIds don't expose an id field for reverse-mapping

Test plan

• Review doc wording in context of the "Convert a Simple Product to Color Variants" and "Important Notes" sections

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