Doc: InitPlans aren't parallel-restricted any more.

Commit e08d74c removed that restriction, but missed updating
the documentation about it.  Noted by Egor Rogov.

Discussion: https://postgr.es/m/cdc8f87b-a378-4e22-6d29-40ae32dd97d1@postgrespro.ru

Author: tglsfdc

doc/src/sgml/parallel.sgml

@@ -510,12 +510,6 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
       </para>
     </listitem>
 
-    <listitem>
-      <para>
-        Plan nodes to which an <literal>InitPlan</literal> is attached.
-      </para>
-    </listitem>
-
     <listitem>
       <para>
         Plan nodes that reference a correlated <literal>SubPlan</literal>.