Fix constants in the documentation

[intjelic]

Constants are defined in classes name space and are not enumeration unlike in C++. I had to work around saying for each method/attribute that returns a constant "a class's constant".

Example: classmethod is_key_pressed(key)

Check if a key is pressed.
Parameters: key (sfml.window.Keyboard‘s constant) – Key to check

I guess there's a more elegant way to handle constant.