ACTION-2034: Add "document" to the glossary

So trying to complete this action, I see that we use the word Document in  
disparate ways.

For example:
  - For the container of the XForm
  - For an instance
  - For an external thing such as a schema.

In the earler discussion  
(https://www.w3.org/2015/10/07-forms-minutes.html#item05), it was said  
"Some implementations allow access to non-instances".

So I'd like to ask what that really means, and what the ramifications  
would be of using "instance", with a note somewhere saying "Some  
implementations allow access to non-instances".

We can discuss this today.

Steven