Update tag documentation:

* Remove latitude/longitude
* Add delete tag API
* Mark create/delete API as "internal"
* Create API returns TRUE, not the created tag

More changes are probably needed - the tag schema is out of date (email has been replaced with owner/actor, by the look of it). But I'm not clear enough on exactly how this works to document it clearly.

I wasn't quite sure what to do with the update API, so I just marked it as internal for now.