Document trigger-firing behavior for inheritance/partitioning.

Amit Langote, reviewed Thomas Munro and by me.

Discussion: http://postgr.es/m/CA+Tgmoadpcs3=mMgdyqVX7L7L_PwO_Dn5j-98a6Tj7ByBuimUQ@mail.gmail.com

doc/src/sgml/trigger.sgml

@@ -122,6 +122,14 @@
     identified as needing to be operated on.
    </para>
 
+   <para>
+    A statement that targets a parent table in a inheritance or partitioning
+    hierarchy does not cause the statement-level triggers of affected child
+    tables to be fired; only the parent table's statement-level triggers are
+    fired.  However, row-level triggers of any affected child tables will be
+    fired.
+   </para>
+
    <para>
     If an <command>INSERT</command> contains an <literal>ON CONFLICT
     DO UPDATE</> clause, it is possible that the effects of all