doc: clearify trigger behavior for inheritance

The previous wording added in PG 10 wasn't specific enough about the
behavior of statement and row triggers when using inheritance.

Reported-by:  <ian@thepathcentral.com>

Discussion: https://postgr.es/m/20171129193934.27108.30796@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/ref/create_trigger.sgml

@@ -501,9 +501,10 @@ UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</
 
   <para>
    Modifying a partitioned table or a table with inheritance children fires
-   statement-level triggers directly attached to that table, but not
+   statement-level triggers attached to the explicitly named table, but not
    statement-level triggers for its partitions or child tables.  In contrast,
-   row-level triggers are fired for all affected partitions or child tables.
+   row-level triggers are fired on the rows in effected partitions or
+   child tables, even if they are not explicitly named in the query.
    If a statement-level trigger has been defined with transition relations
    named by a <literal>REFERENCING</literal> clause, then before and after
    images of rows are visible from all affected partitions or child tables.