parallel.sgml for Gather with InitPlans

In the wake of commit e89a71fb449af2ef74f47be1175f99956cf21524,
parallel.sgml is no longer correct about the effect of InitPlans:

  <para>
    The following operations are always parallel restricted.
  </para>

...

      <para>
        Access to an <literal>InitPlan</literal> or correlated
<literal>SubPlan</literal>.
      </para>

I thought about this a bit and came up with the attached patch.  Other ideas?

-- 
Robert Haas
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