IBM-DM-020: Sharing of namespace nodes

Data Model Section 6.4.1 (Namespace Nodes--Overview): The last sentence of 
the last paragraph states that "Elements never share namespace nodes." It 
should be clearly stated that this rule applies only to implementations 
that expose the namespace axis. For implementations that do not expose 
this (deprecated) axis, sharing namespace nodes is a very reasonable 
strategy. For example, in Appendix D, just before the box containing 
Document Node D1, we read, "We're assuming an implementation that does not 
expose the namespace axis. Therefore we can share namespace nodes across 
multiple elements."