Update docstrings in mathfu::Quaternion to accurately reflect return types. Also, add some TODOs about removing non-const reference access to quaternion components, which will be not-well-defined once quaternions are backed by a SIMD compatible store.
Update docstrings in mathfu::Quaternion to accurately reflect return types.
Also, add some TODOs about removing non-const reference access to quaternion components, which will be not-well-defined once quaternions are backed by a SIMD compatible store.

PiperOrigin-RevId: 222845007
Change-Id: I2accc330046dfddc9f722b1770107b8e7e7e9687

google/mathfu,google/mathfu,google/mathfu