documentation of conref restrictions

["Bruce Nevin (bnevin)" <bnevin@cisco.com>]

This was nagging at me from the minutes for September 29:

> Su-Laine stated that she thinks most people are not aware of how many
> restrictions on conref exist in DITA 1.1, since most editors 
> do not check for them.

Should the constraints be summarized in a prominent way in some unified
place? I see

- Introduction to DITA / Basic concepts / Content Reuse

- DITA Markup / Linking and addressing 
[URI and keyref but no mention of conref. TOC title diverges from topic
title "DITA linking and addressing"]

- DITA Markup / Metadata elements and common attributes / Common
attributes / Identifier and content referencing attributes 
[virtually zero information here, and no links to related topics]

- DITA processing / Content inclusion (conref)
[Note that the TOC title, "Content inclusion (conref)", diverges from
the topic title, "Content reference".] 

- Configuration and Extension / Constraint domains / Conref and
generalization for constraint domains.

Am I missing someplace in the arch spec where conref restrictions are
explained all in one place?

	/Bruce