Clarify the nature of schema placeholder keywords

Acknowledge that "$defs" and placeholders like it also indicate
that their values are schemas, which is relevant to whether
they can be detected as a proper "$ref" target or place where
"$id" may appear.

This ensures that similar extension keywords are understood
to have the same concerns as extension applicator keywords.

Author: handrews

jsonschema-core.xml

@@ -1789,7 +1789,9 @@
                     <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
+                            applicator keywords, or with placeholder keywords such as
+                            <xref target="defs">"$defs"</xref> that take one or more subschemas
+                            as a value.  These keywords may be "$defs" and the standard applicators
                             from this document, or extension keywords from a known vocabulary, or
                             implementation-specific custom keywords.
                         </t>
@@ -1911,7 +1913,7 @@
                     </section>
                 </section>
 
-                <section title='Schema Re-Use With "$defs"'>
+                <section title='Schema Re-Use With "$defs"' anchor="defs">
                     <t>
                         The "$defs" keyword provides a standardized location for schema
                         authors to inline re-usable JSON Schemas into a more general schema.