[1043] Clarify last paragraph in "contains" section

Also fixes the "validates" tense to match the first use.

Author: ssilverman

jsonschema-core.xml

@@ -2352,10 +2352,10 @@
                         <t>
                             This keyword produces an annotation value which is an array of
                             the indexes to which this keyword validates successfully when applying
-                            its subschema, in ascending order. The value MAY be a boolean "true" if the
-                            subschema validated successfully when applied to every index of the instance.
-                            If the instance array this keywords subschema is applicable to is empty,
-                            the annotation value MUST NOT be missing.
+                            its subschema, in ascending order. The value MAY be a boolean "true" if
+                            the subschema validates successfully when applied to every index of the
+                            instance. The annotation MUST be present if the instance array to which
+                            this keyword's schema applies is empty.
                         </t>
                     </section>
                 </section>