Improve user-facing JSON documentation.

I started out with the intention of just fixing the info about the jsonb
operator classes, but soon found myself copy-editing most of the JSON
material.  Hopefully it's more readable now.

doc/src/sgml/gin.sgml

@@ -416,7 +416,7 @@
  <para>
   Of the two operator classes for type <type>jsonb</>, <literal>jsonb_ops</>
   is the default.  <literal>jsonb_hash_ops</> supports fewer operators but
-  will work with larger indexed values than <literal>jsonb_ops</> can support.
+  offers better performance for those operators.
  </para>
 
 </sect1>

doc/src/sgml/release-9.3.sgml

@@ -3618,14 +3618,14 @@ ALTER EXTENSION hstore UPDATE;
       <listitem>
        <para>
         Allow <type>JSON</> values to be <link
-        linkend="functions-json-table">converted into records</link>
+        linkend="functions-json">converted into records</link>
         (Andrew Dunstan)
        </para>
       </listitem>
 
       <listitem>
        <para>
-        Add <link linkend="functions-json-table">functions</link> to convert
+        Add <link linkend="functions-json">functions</link> to convert
         scalars, records, and <type>hstore</> values to <type>JSON</> (Andrew
         Dunstan)
        </para>