[Docs] Clarify Optional Sample Data

[tjwiebell]

Description

re: https://magentocommeng.slack.com/archives/C71HNKYS2/p1591716658341100

Clarify that sample data is optional, reduce any confusion for first time installations that have their own data.

Related Issue

Closes #ISSUE_NUMBER.

Acceptance

Verification Stakeholders

Specification

Verification Steps

1. Verify my english is guud

Screenshots / Screen Captures (if appropriate)

Checklist

• I have updated the documentation accordingly, if necessary.

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Performance Test Results

The following fails have been reported by WebpageTest. These numbers indicates a possible performance issue with the PR which requires further manual testing to validate.

https://pr-2474.pwa-venia.com : LH Performance Expected 0.85 Actual 0.52, LH Best Practices Expected 1 Actual 0.92
https://pr-2474.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.36, LH Best Practices Expected 1 Actual 0.92
https://pr-2474.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.5, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92