Document behavior of the .** jsonpath accessor in the lax mode

When the .** jsonpath accessor handles the array, it selects both array and
each of its elements.  When using lax mode, subsequent accessors automatically
unwrap arrays.  So, the content of each array element may be selected twice.

Even though this behavior is counterintuitive, it's correct because everything
works as designed.  This commit documents it.

Backpatch to 12 where the jsonpath language was introduced.

Reported-by: Thomas Kellerer
Bug: #16828
Discussion: https://postgr.es/m/16828-2b0229babfad2d8c%40postgresql.org
Discussion: https://postgr.es/m/CAPpHfdtS-nNidT%3DEqZbAYOPcnNOWh_sd6skVdu2CAQUGdvpT8Q%40mail.gmail.com
Author: Alexandex Korotkov, revised by Tom Lane
Reviewed-by: Alvaro Herrera, Thomas Kellerer, Tom Lane
Backpatch-through: 12

Author: akorotkov

doc/src/sgml/func.sgml

@@ -13149,6 +13149,24 @@ table2-mapping
 </programlisting>
    </para>
 
+   <para>
+    The <literal>.**</literal> accessor can lead to surprising results
+    when using the lax mode. For instance, the following query selects every
+    <literal>HR</literal> value twice:
+<programlisting>
+lax $.**.HR
+</programlisting>
+    This happens because the <literal>.**</literal> accessor selects both
+    the <literal>segments</literal> array and each of its elements, while
+    the <literal>.HR</literal> accessor automatically unwraps arrays when
+    using the lax mode. To avoid surprising results, we recommend using
+    the <literal>.**</literal> accessor only in the strict mode. The
+    following query selects each <literal>HR</literal> value just once:
+<programlisting>
+strict $.**.HR
+</programlisting>
+   </para>
+
    </sect3>
 
    <sect3 id="jsonpath-regular-expressions">