Clarify angle units in the documentation

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()`](https://docs.rs/nalgebra/latest/nalgebra/geometry/struct.Rotation.html#method.from_euler_angles) and [`Rotation::euler_angles()`](https://docs.rs/nalgebra/latest/nalgebra/geometry/struct.Rotation.html#method.euler_angles) but [a quick search](https://docs.rs/nalgebra/latest/nalgebra/all.html?search=angle) showed other cases.