doc: Explain that ereport doesn't return for ERROR or higher levels.

Christophe Pettus

doc/src/sgml/sources.sgml

@@ -142,6 +142,13 @@ ereport(ERROR,
     a message text.  Also, an optional <quote>hint</> message is provided.
    </para>
 
+   <para>
+    If the severity level is <literal>ERROR</> or higher,
+    <function>ereport</> aborts the execution of the user-defined
+    function and does not return to the caller. If the severity level is
+    lower than <literal>ERROR</>, <function>ereport</> returns normally.
+   </para>
+    
    <para>
     The available auxiliary routines for <function>ereport</> are:
   <itemizedlist>