OP Identifier vs. OP-Specific Identifier

[Recordon, David]

So I'm working on cleaning up the terminology section with edits from
Drummond.  On first read I had no idea what the difference between "OP
Identifier" and "OP-Specific Identifier" were.  Now that my brain has
kicked in I do, but I have the feeling this is going to be really
confusing for others reading the specification.

I really think we need to get our terminology down, since right now it
is honestly quite confusing:
 - Identifier (umbrella definition)
 - Claimed Identifier (umbrella definition)
 - OP-Specific Identifier (needs context)
 - OP Identifier (needs context)
 - Public Identifier (tries to create context)
 - Private Identifier (tries to create context)
 - Privacy-protected login (have we even defined this)

How can we clean this up?  I think the problem is that we're trying to
describe features via the terminology section.

--David