Make some more improvements to parallel query documentation.

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