Make some more improvements to parallel query documentation.

Many places that mentioned only Gather should also mention Gather
Merge, or should be phrased in a more neutral way.  Be more clear
about the fact that max_parallel_workers_per_gather affects the number
of workers the planner may want to use.  Fix a typo.  Explain how
Gather Merge works.  Adjust wording around parallel scans to be a bit
more clear.  Adjust wording around parallel-restricted operations for
the fact that uncorrelated subplans are no longer restricted.

Patch by me, reviewed by Erik Rijkers

Discussion: http://postgr.es/m/CA+TgmoZsTjgVGn=ei5ht-1qGFKy_m1VgB3d8+Rg304hz91N5ww@mail.gmail.com

doc/src/sgml/config.sgml

@@ -2050,8 +2050,8 @@ include_dir 'conf.d'
        <listitem>
         <para>
          Sets the maximum number of workers that can be started by a single
-         <literal>Gather</literal> node.  Parallel workers are taken from the
-         pool of processes established by
+         <literal>Gather</literal> or <literal>Gather Merge</literal> node.
+         Parallel workers are taken from the pool of processes established by
          <xref linkend="guc-max-worker-processes">, limited by
          <xref linkend="guc-max-parallel-workers">.  Note that the requested
          number of workers may not actually be available at run time.  If this