Clarify vocab specs needn't be formal or published.

Author: handrews

jsonschema-core.xml

@@ -399,6 +399,13 @@
                         of any vocabulary, there is no analogous mechanism to indicate individual
                         keyword usage.
                     </t>
+                    <t>
+                        A schema vocabulary can be defined by anything from an informal description
+                        to a standards proposal, depending on the audience and interoperability
+                        expectations.  In particular, in order to facilitate vocabulary use within
+                        non-public organizations, a vocabulary specification need not be published
+                        outside of its scope of use.
+                    </t>
                 </section>
                 <section title="Meta-Schemas">
                     <t>