Fix description for $varname jsonpath variable

The description is ended part way and PASSING clause is not implemented yet.
But the variables might be passed as parameters to several jsonpath functions.
So, complete the description based on the current implementation, leaving
description of PASSING clause in TODO.

Discussion: https://postgr.es/m/CAKPRHz%2BxOuQSSvkuB1mCQjedd%2BB2B1Vnkrq0E-pLmoXyTO%2Bz9Q%40mail.gmail.com
Author: Kyotaro Horiguchi, Alexander Korotkov

doc/src/sgml/json.sgml

@@ -733,10 +733,12 @@ SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @> '{"tags": ["qu
      </row>
      <row>
       <entry><literal>$varname</literal></entry>
-      <entry>A named variable. Its value must be set in the
-      <command>PASSING</command> clause of an SQL/JSON query function.
- <!-- TBD: See <xref linkend="sqljson-input-clause"/> -->
-      for details.
+      <entry>
+        A named variable. Its value can be set by the parameter
+        <parameter>vars</parameter> of several JSON processing functions.
+        See <xref linkend="functions-json-processing-table"/> and
+        its notes for details.
+        <!-- TODO: describe PASSING clause once implemented !-->
       </entry>
      </row>
      <row>