Improve wording: define additional vocabs/keywords

Make it more clear that extending involves defining more
vocabularies and/or keywords, not adding existing
vocabularies/keywords from some place to some other place.

Author: handrews

jsonschema-core.xml

@@ -154,10 +154,11 @@
                 consists of a list of keywords, together with their syntax and semantics.
             </t>
             <t>
-                JSON Schema can be extended either by adding vocabularies, or less formally
-                by adding keywords without defining a vocabulary.  Unrecognized individual
-                keywords are ignored, while the behavior with respect to an unrecognized
-                vocabulary can be controlled when declaring which vocabularies are in use.
+                JSON Schema can be extended either by defining additional vocabularies,
+                or less formally by defining additional keywords outside of any vocabulary.
+                Unrecognized individual keywords are ignored, while the behavior with respect
+                to an unrecognized vocabulary can be controlled when declaring which
+                vocabularies are in use.
             </t>
             <t>
                 This document defines a core vocabulary that MUST be supported by any