Add docs about referencing subassemlies.

[adam-urbanczyk]

Using partX/subpartY@... is not mentioned anywhere in the docs:

assy2 = (
        cq.Assembly()
        .add(subassembly, name="partX", color=cq.Color("orange"))
        .add(part2, name="part2", color=cq.Color("red"))

        #NB: partX/subpartX1
        .constrain("partX/subpartX1@faces@>X", "part2?right", "Plane")
)
assy2.solve()