More work on the JSON/JSONB user documentation.

Document existence operator adequately; fix obsolete claim that no
Unicode-escape semantic checks happen on input (it's still true for
json, but not for jsonb); improve examples; assorted wordsmithing.

doc/src/sgml/gin.sgml

@@ -417,6 +417,7 @@
   Of the two operator classes for type <type>jsonb</>, <literal>jsonb_ops</>
   is the default.  <literal>jsonb_hash_ops</> supports fewer operators but
   offers better performance for those operators.
+  See <xref linkend="json-indexing"> for details.
  </para>
 
 </sect1>