Re-allow DISTINCT in pl/pgsql expressions.

I'd omitted this from the grammar in commit c9d529848, figuring that
it wasn't worth supporting.  However we already have one complaint,
so it seems that judgment was wrong.  It doesn't require a huge
amount of code, so add it back.  (I'm still drawing the line at
UNION/INTERSECT/EXCEPT though: those'd require an unreasonable
amount of grammar refactoring, and the single-result-row restriction
makes them near useless anyway.)

Also rethink the documentation: this behavior is a property of
all pl/pgsql expressions, not just assignments.

Discussion: https://postgr.es/m/20210122134106.e94c5cd7@mail.verfriemelt.org

doc/src/sgml/plpgsql.sgml

@@ -917,6 +917,24 @@ PREPARE <replaceable>statement_name</replaceable>(integer, integer) AS SELECT $1
      they are useful to know when trying to diagnose a problem.
      More information appears in <xref linkend="plpgsql-plan-caching"/>.
     </para>
+
+    <para>
+     Since an <replaceable>expression</replaceable> is converted to a
+     <literal>SELECT</literal> command, it can contain the same clauses
+     that an ordinary <literal>SELECT</literal> would, except that it
+     cannot include a top-level <literal>UNION</literal>,
+     <literal>INTERSECT</literal>, or <literal>EXCEPT</literal> clause.
+     Thus for example one could test whether a table is non-empty with
+<programlisting>
+IF count(*) &gt; 0 FROM my_table THEN ...
+</programlisting>
+     since the <replaceable>expression</replaceable>
+     between <literal>IF</literal> and <literal>THEN</literal> is parsed as
+     though it were <literal>SELECT count(*) &gt; 0 FROM my_table</literal>.
+     The <literal>SELECT</literal> must produce a single column, and not
+     more than one row.  (If it produces no rows, the result is taken as
+     NULL.)
+    </para>
   </sect1>
 
   <sect1 id="plpgsql-statements">
@@ -973,20 +991,6 @@ my_array[1:3] := array[1,2,3];
 complex_array[n].realpart = 12.3;
 </programlisting>
     </para>
-
-    <para>
-     It's useful to know that what follows the assignment operator is
-     essentially treated as a <literal>SELECT</literal> command; as long
-     as it returns a single row and column, it will work.  Thus for example
-     one can write something like
-<programlisting>
-total_sales := sum(quantity) from sales;
-</programlisting>
-     This provides an effect similar to the single-row <literal>SELECT
-     ... INTO</literal> syntax described in
-     <xref linkend="plpgsql-statements-sql-onerow"/>.  However, that syntax
-     is more portable.
-    </para>
    </sect2>
 
    <sect2 id="plpgsql-statements-sql-noresult">