Split "Metadata" in to re-use + annotations

In preparation for the applicability text, split out "definitions"
into its own section about re-use, and rename the remainder of
the old metadata section to focus on annotation of schemas with
keywords or new vocabularies.

Rework the "definitoins" section text, but leave the other
sections alone until the next commit.

Author: handrews

jsonschema-validation.xml

@@ -1100,25 +1100,23 @@
 
         </section>
 
-        <section title="Metadata keywords">
-            <section title="definitions">
-                <t>
-                    This keyword's value MUST be an object.
-                    Each member value of this object MUST be a valid JSON Schema.
-                </t>
-                <t>
-                    This keyword plays no role in validation per se. Its role is to provide
-                    a standardized location for schema authors to inline JSON Schemas into a
-                    more general schema.
-                </t>
-
-                <t>
-                    As an example, here is a schema describing an array of positive
-                    integers, where the positive integer constraint is a subschema in
-                    "definitions":
+        <section title='Schema re-use with "definitions"'>
+            <t>
+                The "definitions" keywords provides a standardized location for schema
+                authors to inline re-usable JSON Schemas into a more general schema.
+                The keyword does not directly affect the validation result.
+            </t>
+            <t>
+                This keyword's value MUST be an object.
+                Each member value of this object MUST be a valid JSON Schema.
+            </t>
+            <t>
+                As an example, here is a schema describing an array of positive
+                integers, where the positive integer constraint is a subschema in
+                "definitions":
 
-                    <figure>
-                        <artwork>
+                <figure>
+                    <artwork>
 <![CDATA[
 {
     "type": "array",
@@ -1131,10 +1129,12 @@
     }
 }
 ]]>
-                        </artwork>
-                    </figure>
-                </t>
-            </section>
+                    </artwork>
+                </figure>
+            </t>
+        </section>
+
+        <section title="Schema annotations and extension vocabularies">
 
             <section title='"title" and "description"'>
                 <t>