Doc: Update description for parallel insert reloption.

Commit c8f78b6161 added a new reloption to enable inserts in parallel-mode
but forgot to update at one of the places about the same in docs. In
passing, fix a typo in the same commit.

Reported-by: Justin Pryzby
Author: Justin Pryzby
Reviewed-by: "Hou, Zhijie", Amit Kapila
Discussion: https://postgr.es/m/20210318025228.GE11765@telsasoft.com

doc/src/sgml/ref/alter_table.sgml

@@ -722,7 +722,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      <para>
       <literal>SHARE UPDATE EXCLUSIVE</literal> lock will be taken for
       fillfactor, toast and autovacuum storage parameters, as well as the
-      planner parameter <varname>parallel_workers</varname> and
+      planner parameters <varname>parallel_workers</varname> and
       <varname>parallel_insert_enabled</varname>.
      </para>
     </listitem>

doc/src/sgml/ref/create_table.sgml

@@ -1338,8 +1338,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     If a table parameter value is set and the
     equivalent <literal>toast.</literal> parameter is not, the TOAST table
     will use the table's parameter value.
-    Specifying these parameters for partitioned tables is not supported,
-    but you may specify them for individual leaf partitions.
+    These parameters, with the exception of
+    <literal>parallel_insert_enabled</literal>, are not supported on partitioned
+    tables, but may be specified for individual leaf partitions.
    </para>
 
    <variablelist>