Improved documentation required for IfcShapeAspect ProductDefinitional attribute

@jmirtsch [in this thread](https://forums.buildingsmart.org/t/productdefinition-attribute-of-ifcshapeaspect/1077) says that the definition does not make much sense, and perhaps some more examples or diagrams might help.

Here's the definition quoted:

> An indication that the shape aspect is on the physical boundary of the product definition shape. If the value of this attribute is TRUE, it shall be asserted that the shape aspect being identified is on such a boundary. If the value is FALSE, it shall be asserted that the shape aspect being identified is not on such a boundary. If the value is UNKNOWN, it shall be asserted that it is not known whether or not the shape aspect being identified is on such a boundary. --- EXAMPLE: Would be FALSE for a center line, identified as shape aspect; would be TRUE for a cantilever. ---

I don't understand it either. Can someone clarify, and then see if rewording can help?

Ping @berlotti @TLiebich @aothms