Define "applicability" and "attachment"

This is a bit more groundwork for the rewrite, and is relevant
to issue json-schema-org#423.

The text here will likely be further restructured for flow and
clarity in the rewrite (and the section title will probably
empthasize the concept more).

Author: handrews

jsonschema-hyperschema.xml

@@ -184,7 +184,12 @@
             <section title="Interaction with validation">
                 <t>
                     Hyper-schema keywords can be applied when the instance is valid against
-                    the schema that includes those keywords.
+                    the schema that includes those keywords.  For any given location in the
+                    instance, each (sub)schema against which it successfully validates is
+                    considered to be <spanx>applicable</spanx> to that location in the
+                    instance.  The location is said to be the (sub)schema's point of
+                    <spanx>attachment</spanx>.  This section provides the specific
+                    rules for determining applicabiity.
                 </t>
                 <t>
                     Hyper-schemas MUST NOT be applied to an instance if the instance fails to