Clarify which relkinds accept column comments.

Per discussion with Josh Kupershmidt.

doc/src/sgml/ref/comment.sgml

@@ -26,7 +26,7 @@ COMMENT ON
   AGGREGATE <replaceable class="PARAMETER">agg_name</replaceable> (<replaceable class="PARAMETER">agg_type</replaceable> [, ...] ) |
   CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>) |
   COLLATION <replaceable class="PARAMETER">object_name</replaceable> |
-  COLUMN <replaceable class="PARAMETER">table_name</replaceable>.<replaceable class="PARAMETER">column_name</replaceable> |
+  COLUMN <replaceable class="PARAMETER">relation_name</replaceable>.<replaceable class="PARAMETER">column_name</replaceable> |
   CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ON <replaceable class="PARAMETER">table_name</replaceable> |
   CONVERSION <replaceable class="PARAMETER">object_name</replaceable> |
   DATABASE <replaceable class="PARAMETER">object_name</replaceable> |
@@ -98,7 +98,7 @@ COMMENT ON
   <variablelist>
    <varlistentry>
     <term><replaceable class="parameter">object_name</replaceable></term>
-    <term><replaceable class="parameter">table_name.column_name</replaceable></term>
+    <term><replaceable class="parameter">relation_name</replaceable>.<replaceable>column_name</replaceable></term>
     <term><replaceable class="parameter">agg_name</replaceable></term>
     <term><replaceable class="parameter">constraint_name</replaceable></term>
     <term><replaceable class="parameter">function_name</replaceable></term>
@@ -111,6 +111,9 @@ COMMENT ON
       aggregates, collations, conversions, domains, foreign tables, functions,
       indexes, operators, operator classes, operator families, sequences,
       text search objects, types, and views can be schema-qualified.
+      When commenting on a column,
+      <replaceable class="parameter">relation_name</replaceable> must refer
+      to a table, view, composite types, or foreign table.
      </para>
     </listitem>
    </varlistentry>