Add intro material to unevaluated applicator vocab

This attempts to address various points of confusion noted
in feedback on the most recent draft.

Author: handrews

jsonschema-core.xml

@@ -2478,6 +2478,28 @@
         </section>
 
         <section title="A Vocabulary for Unevaluated Locations">
+            <t>
+                The purpose of these keywords is to enable schema authors to apply
+                subschemas to array items or object properties that have not been
+                successfully evaluated against any dynamic-scope subschema of any
+                adjacent keywords.
+            </t>
+            <t>
+                These instance items or properties may have been unsuccessfully evaluated
+                against one or more adjacent keyword subschemas, such as when an assertion
+                in a branch of an "anyOf" fails.  Such failed evaluations are not considered
+                to contribute to whether or not the item or property has been evaluated.
+                Only successful evaluations are considered.
+            </t>
+            <t>
+                Recall that adjacent keywords are keywords within the same schema object,
+                and that the dynamic-scope subschemas include reference targets as well as
+                lexical subschemas.
+            </t>
+            <t>
+                The behavior of these keywords depend on the annotation results of
+                adjacent keywords that apply to the instance location being validated.
+            </t>
             <t>
                 Meta-schemas that do not use "$vocabulary" SHOULD be considered to
                 require this vocabulary as if its URI were present with a value of true.