Clarify angle units in the documentation

[videbar]

Some of the functions refer to angles, but they don't specify that these angels are to be expressed in radians. I think it'd be helpful to add it to the documentation. I've noticed it on Rotation::from_euler_angles() and Rotation::euler_angles() but a quick search showed other cases.

[videbar]

I've been thinking about this and I believe it would also be good to specify the basis used for the roll, pitch and yaw angles. From my testing it seems that it's the extrinsic basis, but it would be great to have confirmation.

[camdenmcgath]

@videbar I can take this issue! I will first clarify that angles are to expressed as radians and move on to your next comment when that's done!