Improve documentation for Quaternion.canonicalizedTransform

[stephentyrone]

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I'd like to make this more precise. If I wasn't concerned about jargon, I would say "the canonical representative of the equivalence class of quaternions representing the same 3d transformation."

Originally posted by @stephentyrone in https://github.com/apple/swift-numerics/pull/124

[stephentyrone]

CC @markuswntr