clarify the requirements to implement the different output formats

Author: karenetheridge

jsonschema-core.xml

@@ -2650,10 +2650,10 @@
                             structure that matches the exact structure of the schema.
                         </t>
                     </list>
-                    An implementation SHOULD provide at least the "flag", "basic", or "detailed"
+                    An implementation SHOULD provide at least one of the "flag", "basic", or "detailed"
                     format and MAY provide the "verbose" format.  If it provides one or more of the
-                    complex formats, it MUST also provide the "flag" format. Implementations SHOULD
-                    specify in their documentation which formats they support.
+                    "detailed" or "verbose" formats, it MUST also provide the "flag" format.
+                    Implementations SHOULD specify in their documentation which formats they support.
                 </t>
 
             </section>