improve docs of vec2d.py by unifying doc styles

[VeriTas-arch]

Contents

• Trying to unify the doc style with imperative-style instead of a combination of it and descriptive-style. see reference here
• Minor changes to examples on code format (i.e. add some space to Vec2d variables, for instance, changing from Vec2d(3,4) to Vec2d(3, 4)).
• Remove a temporary variable (projected_length_times_other_length, line 400). (Such variables may initially exist given the readability of codes, yet seeming unnecessary under some conditions, such as several well-known vector operations).

Summary

Minor changes only.

[viblo]

Thanks for the PR!