Exclude LDO schema keywords from applicability

When discovering links for a hyper-schema, it does not make
sense to examine the schema keywords within the discovered links.

While this was already the case as there are no rules for
determining the applicability of those keywords to an instance,
that is not obvious so make it explicit.

Author: handrews

jsonschema-hyperschema.xml

@@ -215,6 +215,12 @@
                         validation specification</xref>.  Links that are not valid with the
                         current instance and/or client input MUST be excluded.
                     </t>
+                    <t>
+                        Schemas that are found as values within Link Description Objects
+                        are not considered to be applicable to the instance, even when the
+                        link is applicable and valid.  They are only applied to various kinds
+                        of input or message payloads as described for each keyword.
+                    </t>
                     <t>
                         These links MUST all be presented as a single set, regardless
                         of the number of subschemas involved.  The order of links in the set