Remove "Internal Referencing" and rework example

We know from feedback that the "internal referencing" concept
and in particular the "same document" wording is confusing and
for some people misleading.  Remove the term and focus the wording
on normal RFC 3986 URI reference resolution.

Cover the same logical ground as "internal referencing" by discussing
whether and how the implementation knows of the identified schema,
using the same general ideas as covered in the previous section
on loading (or not loading) referenced schemas.

Author: handrews

jsonschema-core.xml

@@ -731,19 +731,19 @@
                         with the same URI, validators SHOULD raise an error condition.
                     </t>
                 </section>
-            </section>
-
-            <section title="Dereferencing">
-                <section title="Internal References">
+                <section title="Dereferencing">
                     <t>
                         Schemas can be identified by any URI that has been given to them, including
-                        a JSON Pointer or their URI given directly by "$id".
+                        a JSON Pointer or their URI given directly by "$id".  In all cases,
+                        dereferencing a "$ref" reference involves first resolving its value as a
+                        URI reference against the current base URI per
+                        <xref target="RFC3986">RFC 3986</xref>.
                     </t>
                     <t>
-                        Tools SHOULD take note of the URIs that schemas, including subschemas,
-                        provide for themselves using "$id". This is known as "Internal referencing".
+                        If the resulting URI identifies a schema within the current document, or
+                        within another schema document that has been made available to the implementation,
+                        then that schema SHOULD be used automatically.
                     </t>
-
                     <t>
                         For example, consider this schema:
                     </t>
@@ -760,7 +760,8 @@
     "definitions": {
         "single": {
             "$id": "#item",
-            "type": "integer"
+            "type": "object",
+            "additionalProperties": { "$ref": "other.json" }
         }
     }
 }
@@ -775,9 +776,22 @@
                     <t>
                         When an implementation then looks inside the &lt;#/items&gt; schema, it
                         encounters the &lt;#item&gt; reference, and resolves this to
-                        &lt;http://example.net/root.json#item&gt; which is understood as the schema
-                        defined elsewhere in the same document without needing to
-                        resolve the fragment against the base URI.
+                        &lt;http://example.net/root.json#item&gt;, which it has seen defined in
+                        this same document and can therefore use automatically.
+                    </t>
+                    <t>
+                        When an implementation encounters the reference to "other.json", it resolves
+                        this to &lt;http://example.net/other.json&gt;, which is not defined in this
+                        document.  If a schema with that identifier has otherwise been supplied to
+                        the implementation, it can also be used automatically.
+                        <cref>
+                            What should implementations do when the referenced schema is not known?
+                            Are there circumstances in which automatic network dereferencing is
+                            allowed?  A same origin policy?  A user-configurable option?  In the
+                            case of an evolving API described by Hyper-Schema, it is expected that
+                            new schemas will be added to the system dynamically, so placing an
+                            absolute requirement of pre-loading schema documents is not feasible.
+                        </cref>
                     </t>
                 </section>
             </section>