doc: Remove event trigger firing matrix

This is difficult to maintain accurately, and it was probably already
somewhat incorrect, especially in the sql_drop and table_rewrite
categories.

The prior section already documented which DDL commands are *not*
supported (which was also slightly outdated), so let's expand that a
bit and just rely on that instead of listing out each command in full
detail.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CACJufxE_UAuxcM08BW5oVsg34v0cFWoEt8yBa5xSAoKLmL6LTQ%40mail.gmail.com

src/backend/commands/event_trigger.c

@@ -1123,7 +1123,7 @@ EventTriggerInvoke(List *fn_oid_list, EventTriggerData *trigdata)
 /*
  * Do event triggers support this object type?
  *
- * See also event trigger support matrix in event-trigger.sgml.
+ * See also event trigger documentation in event-trigger.sgml.
  */
 bool
 EventTriggerSupportsObjectType(ObjectType obtype)
@@ -1147,7 +1147,7 @@ EventTriggerSupportsObjectType(ObjectType obtype)
 /*
  * Do event triggers support this object class?
  *
- * See also event trigger support matrix in event-trigger.sgml.
+ * See also event trigger documentation in event-trigger.sgml.
  */
 bool
 EventTriggerSupportsObject(const ObjectAddress *object)

src/backend/tcop/utility.c

@@ -535,6 +535,9 @@ ProcessUtility(PlannedStmt *pstmt,
  * event trigger code not be invoked when doing START TRANSACTION for
  * example, because we might need to refresh the event trigger cache,
  * which requires being in a valid transaction.
+ *
+ * When adding or moving utility commands, check that the documentation in
+ * event-trigger.sgml is kept up to date.
  */
 void
 standard_ProcessUtility(PlannedStmt *pstmt,