IBM-DM-002: Node properties need better definitions

In the Data Model document, each kind of node has a list of named 
properties, but the document does not in general specify the information 
that is contained in these properties. In some cases, the properties are 
directly associated with accessor functions from which their types can be 
deduced (for example, dm:nilled() returns a boolean, so presumably the 
nilled property contains a boolean). But in other cases it is not so 
clear. Some essential information, such as the "tuple" representation of 
dates and times, does not seem to be present in the node properties. The 
content of the various properties should be clearly explained (and 
enhanced, where necessary to support the accessor functions). The types of 
the properties should be declared where appropriate (some of them may be 
implementation-dependent).