Incorporate awwright's suggestions on cref #1

Better explanation of the concerns around and possible future
direction of $schema usage.

Also add an XML comment to remind us of the most notable concern,
so we can skip the whole thing where we forget about it and then
argue about it until we work it out again.  Which of course has
never happened...

jsonschema-core.xml

@@ -436,10 +436,21 @@
             </t>
             <t>
                 <cref>
-                    While this pattern is likely to remain best practice for schema authoring,
-                    implementation behavior is subject to be revised or liberalized in future
-                    drafts.
+                    Using multiple "$schema" keywords in the same document would imply that the
+                    vocabulary and therefore behavior can change within a document.  This would
+                    necessitate resolving a number of implementation concerns that have not yet
+                    been clearly defined.  So, while the pattern of using "$schema" only in root
+                    schemas is likely to remain the best practice for schema authoring,
+                    implementation behavior is subject to be revised or liberalized in
+                    future drafts.
                 </cref>
+                <!--
+                    In particular, the process of validating an instance, including validating a
+                    schema as an instance against its meta-schema, only allows for a single set
+                    of rules across the entire instance document.  There is no equivalent of
+                    changing the meta-schema partway through the validation for non-schema
+                    instances.
+                  -->
             </t>
             <t>
                 Values for this property are defined in other documents and by other parties.