doc: Add ALTER/DROP ROUTINE to the event trigger matrix

ALTER ROUTINE triggers the events ddl_command_start and ddl_command_end,
and DROP ROUTINE triggers sql_drop, ddl_command_start and
ddl_command_end, but this was not mention on the matrix table.

Reported-by: Leslie Lemaire
Discussion: https://postgr.es/m/164647533363.646.5802968483136493025@wrigleys.postgresql.org
Backpatch-through: 11

doc/src/sgml/event-trigger.sgml

@@ -293,6 +293,14 @@
         <entry align="center"><literal>-</literal></entry>
         <entry align="left"></entry>
        </row>
+       <row>
+        <entry align="left"><literal>ALTER ROUTINE</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="left"></entry>
+       </row>
        <row>
         <entry align="left"><literal>ALTER SCHEMA</literal></entry>
         <entry align="center"><literal>X</literal></entry>
@@ -853,6 +861,14 @@
         <entry align="center"><literal>-</literal></entry>
         <entry align="left"></entry>
        </row>
+       <row>
+        <entry align="left"><literal>DROP ROUTINE</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="left"></entry>
+       </row>
        <row>
         <entry align="left"><literal>DROP RULE</literal></entry>
         <entry align="center"><literal>X</literal></entry>