`waist_sizes` in `AstigmaticGaussianBeam` is confusing

[tomflexcompute]

A user was confused by the name waist_sizes in AstigmaticGaussianBeam and thought it meant diameter instead of radius. To clarify this, we can either enhance the API reference or change waist_sizes to something like waist_radii with backward compatibility preserved.

[momchil-flex]

Well, I assigned @dbochkov-flexcompute , but then saw the good first issue tag, so maybe we can leave it for now until someone needs a good first issue. :)