doc: add examples for array_length() and jsonb_array_length()

The examples show the output of array_length() and jsonb_array_length()
for empty arrays.

Discussion: https://postgr.es/m/CAKFQuwaoBmRuWdMLzLHDCFDJDX3wvfQ7egAF0bpik_BFgG1KWg@mail.gmail.com

Author: David G. Johnston

Backpatch-through: 13

doc/src/sgml/func.sgml

@@ -15446,6 +15446,10 @@ table2-mapping
        <para>
         <literal>json_array_length('[1,2,3,{"f1":1,"f2":[5,6]},4]')</literal>
         <returnvalue>5</returnvalue>
+       </para>
+       <para>
+        <literal>jsonb_array_length('[]')</literal>
+        <returnvalue>0</returnvalue>
        </para></entry>
       </row>
 
@@ -17887,10 +17891,19 @@ SELECT NULLIF(value, '(none)') ...
        </para>
        <para>
         Returns the length of the requested array dimension.
+        (Produces NULL instead of 0 for empty or missing array dimensions.)
        </para>
        <para>
         <literal>array_length(array[1,2,3], 1)</literal>
         <returnvalue>3</returnvalue>
+       </para>
+       <para>
+        <literal>array_length(array[]::int[], 1)</literal>
+        <returnvalue>NULL</returnvalue>
+       </para>
+       <para>
+        <literal>array_length(array['text'], 2)</literal>
+        <returnvalue>NULL</returnvalue>
        </para></entry>
       </row>