Edits from Nico's review

Also a typo fix.

Note that the changes to the sample code are not yet integrated
into the artwork in the XML file.
Try listing out protocol requirements
Add some more security considerations

From Nico's comments.
Mention authz in the body text, if not in the sample code for now
Remove extra whitespace
Mention naming extensions with authz checks
Rework partial context message protection text
Supply stub authorization checking code

(Always returns success, given the 'stub' nature.)
Appease Greg and remove notes about non-local resources

The parenthetical is for a rather uncommon case, I suppose.
Another crack at using partially complete contexts

We probably should be discouraging it, really.
It's very hard to get right.
Attempt to refresh process_context_token discussion

To match the consensus achieved on the list.