Documentation issue: @return technically not correct for out param

[Andos]

Minor documentation issue: Use of the @return command is generating warnings in clang since the function return type is 'void' so it doesn't technically return anything. It is more of an "out" param.

Lines in question: https://github.com/erincatto/box2d/blob/master/include/box2d/b2_body.h#L245-L246

    /// Get the mass data of the body.
    /// @return a struct containing the mass, inertia and center of the body.
    void GetMassData(b2MassData* data) const;