Docs: add example clarifying use of nested JSON containment.

Show how this can be used in practice to make queries simpler and more
flexible.  Also, draw an explicit contrast to the existence operator,
which doesn't work that way.

Peter Geoghegan and Tom Lane

doc/src/sgml/json.sgml

@@ -349,6 +349,34 @@ SELECT '"foo"'::jsonb ? 'foo';
     need to be searched linearly.
   </para>
 
+  <tip>
+   <para>
+    Because JSON containment is nested, an appropriate query can skip
+    explicit selection of sub-objects.  As an example, suppose that we have
+    a <structfield>doc</> column containing objects at the top level, with
+    most objects containing <literal>tags</> fields that contain arrays of
+    sub-objects.  This query finds entries in which sub-objects containing
+    both <literal>"term":"paris"</> and <literal>"term":"food"</> appear,
+    while ignoring any such keys outside the <literal>tags</> array:
+<programlisting>
+SELECT doc-&gt;'site_name' FROM websites
+  WHERE doc @&gt; '{"tags":[{"term":"paris"}, {"term":"food"}]}';
+</programlisting>
+    One could accomplish the same thing with, say,
+<programlisting>
+SELECT doc-&gt;'site_name' FROM websites
+  WHERE doc-&gt;'tags' @&gt; '[{"term":"paris"}, {"term":"food"}]';
+</programlisting>
+    but that approach is less flexible, and often less efficient as well.
+   </para>
+
+   <para>
+    On the other hand, the JSON existence operator is not nested: it will
+    only look for the specified key or array element at top level of the
+    JSON value.
+   </para>
+  </tip>
+
   <para>
     The various containment and existence operators, along with all other
     JSON operators and functions are documented