Better documentation of types of properties

[dgarijo]

Right now it is not straightforward to know how to link two classes with an object property. There are functional, symmetric and other types of object properties listed, but it is not clear that the plain object property can be used if users are not sure what to use.

I think object property should be listed first and have something like (default) to indicate that if you don't know which property to use, you shall use this one.