Document effect of constant folding on CASE.

Back-patch to all supported versions.

Laurenz Albe

doc/src/sgml/func.sgml

@@ -9136,6 +9136,16 @@ SELECT a,
 SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
 </programlisting>
    </para>
+
+   <note>
+    <para>
+     As described in <xref linkend="xfunc-volatility">, functions and
+     operators marked <literal>IMMUTABLE</literal> can be evaluated when
+     the query is planned rather than when it is executed.  This means
+     that constant parts of a subexpression that is not evaluated during
+     query execution might still be evaluated during query planning.
+    </para>
+   </note>
   </sect2>
 
   <sect2>