Update documentation about pseudo-types.

Tone down an overly strong statement about which pseudo-types PLs are
likely to allow.  Add "event_trigger" to the list, as well as
"pg_ddl_command" in 9.5/HEAD.  Back-patch to 9.3 where event_trigger
was added.

Author: tglsfdc

doc/src/sgml/datatype.sgml

@@ -4561,6 +4561,10 @@ SELECT * FROM pg_attribute
     <primary>trigger</primary>
    </indexterm>
 
+   <indexterm zone="datatype-pseudo">
+    <primary>event_trigger</primary>
+   </indexterm>
+
    <indexterm zone="datatype-pseudo">
     <primary>language_handler</primary>
    </indexterm>
@@ -4665,14 +4669,19 @@ SELECT * FROM pg_attribute
 
        <row>
         <entry><type>record</></entry>
-        <entry>Identifies a function returning an unspecified row type.</entry>
+        <entry>Identifies a function taking or returning an unspecified row type.</entry>
        </row>
 
        <row>
         <entry><type>trigger</></entry>
         <entry>A trigger function is declared to return <type>trigger.</></entry>
        </row>
 
+       <row>
+        <entry><type>event_trigger</></entry>
+        <entry>An event trigger function is declared to return <type>event_trigger.</></entry>
+       </row>
+
        <row>
         <entry><type>void</></entry>
         <entry>Indicates that a function returns no value.</entry>
@@ -4695,10 +4704,11 @@ SELECT * FROM pg_attribute
 
    <para>
     Functions coded in procedural languages can use pseudo-types only as
-    allowed by their implementation languages.  At present the procedural
-    languages all forbid use of a pseudo-type as argument type, and allow
+    allowed by their implementation languages.  At present most procedural
+    languages forbid use of a pseudo-type as an argument type, and allow
     only <type>void</> and <type>record</> as a result type (plus
-    <type>trigger</> when the function is used as a trigger).  Some also
+    <type>trigger</> or <type>event_trigger</> when the function is used
+    as a trigger or event trigger).  Some also
     support polymorphic functions using the types <type>anyelement</>,
     <type>anyarray</>, <type>anynonarray</>, <type>anyenum</>, and
     <type>anyrange</>.