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Updating the Pixel Wrangler docs section on extending the rotation. Making them
more general for modifying the rotation. With a portion about swapping.

Change-Id: Ief4c3ffc86f87fe924e94900c16b017f38e9ed80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037327Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738363}