Vector3 IEquatable and Shape documentation

In previous pull request I implemented IEquatable to Vector2 structs, but I forgot to implement it to Vector3, so I did now. Also I updated shapes documentation because on SFML thread I saw that in SFML.Net it is not updated (https://github.com/LaurentGomila/SFML/commit/e9243b7ef69083cce2fcb0651e45e42c1ea7adae).