Adds documentation for antialiasing with impeller

[gaaclarke]

Description of what this PR is changing or adding, and why:

Issues fixed by this PR (if any):

flutter/flutter#183043

PRs or commits this PR depends on (if any):

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[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 68e69c5):

https://flutter-docs-prod--pr13370-antialias-doc-17a96hc6.web.app