Improve documentation for Quaternion.canonicalizedTransform

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_

Locate the documentation for Quaternion.canonicalizedTransform and compare its current wording with the precision requested in the issue. Update the description to explain that it is the canonical representative of the equivalence class of quaternions representing the same 3D transformation, while keeping the terminology understandable.

Written by the indexing model from the issue text.