use cases/req updates

[Bruce D'Arcus <bruce.darcus@OpenDocument.us>]

FYI, I just spent some time updating the use cases and requirement doc:

<http://wiki.oasis-open.org/office/ 
OpenDocumet_Metadata_Use_Cases_and_Requirements>

A little bit of new content, but mostly just adding some structure to  
the page (ToC, internal links, etc.).

I did subtly change the definition of metadata from "data about data"  
to "structured data about data."  Borrowed from the SVG metadata spec.  
Also, I split off a separate "Why is Metadata Important?" section.

I think the document is in good shape, though it could be filled out a  
bit in places. Also, we need to provide the link to the more extensive  
use case doc once we have the final URL for it.

Bruce