PIDF draft mustUnderstand should be schema boolean

The value for the mustUnderstand attribute is currently "Yes" or "No".
Since the meaning is a boolean, it would be better to use a boolean as
defined  by XML Schema.  That would also make use consistent with how SOAP
uses it in version 1.2 where it is a boolean.

If this change was accepted, the values would be 0, 1, true, or false.  

 <!-- Global Attributes -->
 <xs:attribute name="mustUnderstand" type="xs:boolean" default="0" />

> **************************
> Wayne J. Carr
> [email protected]
> 1-503-264-7555
> Intel Labs
> ************************** 
> 
> 


  [reminder: [email protected] for non-technical discussions, please]