Note undefined behavior with unknown $ref targets

The most common situation here, which are basically alternate
names for "definitions" or "$defs", will generally work, but
this makes clear that there is no obligation to attempt to
guarantee the proper behavior in such situations.

We might want to narrow this behavior down in the future,
but for now this at least clearly removes any possible burdensome
and totally impractical inferencing requirements.

Author: handrews

jsonschema-core.xml

@@ -1540,6 +1540,29 @@
                     </t>
                 </section>
 
+                <section title="References to Possible Non-Schemas">
+                    <t>
+                        Subschema objects (or booleans) are recognized by their use with known
+                        applicator keywords.  These keywords may be the standard applicators
+                        from this document, or extension keywords from a known vocabulary or
+                        implementation-specific custom keywords.
+                    </t>
+                    <t>
+                        Multi-level structures of unknown keywords are capable of introducing
+                        nested subschemas, which would be subject to the processing rules for
+                        "$id".  Therefore, having a reference target in such an unrecognized
+                        structure cannot be reliably implemented, and the resulting behavior
+                        is undefined.
+                    </t>
+                    <t>
+                        Note that single-level custom keywords with identical syntax and
+                        semantics to "$defs" do not allow for any intervening "$id" keywords,
+                        and therefore will behave correctly under implementations that attempt
+                        to use any reference target as a schema.  However, this behavior is
+                        implementation-specific and MUST NOT be relied upon for interoperability.
+                    </t>
+                </section>
+
                 <section title="Loading a referenced schema">
                     <t>
                         The use of URIs to identify remote schemas does not necessarily mean anything is downloaded,

jsonschema-validation.xml

@@ -864,8 +864,9 @@
                     </artwork>
                     <postamble>
                         Instances described by this schema should be strings containing HTML, using
-                        whatever character set the JSON string was decoded into (default is
-                        Unicode).
+                        whatever character set the JSON string was decoded into.  Per section 8.1 of
+                        <xref target="RFC8259">RFC 8259</xref>, outside of an entirely closed
+                        system, this MUST be UTF-8.
                     </postamble>
                 </figure>