parallel documentation improvements

Hi,

In commit 054637d2e08cda6a096f48cc99696136a06f4ef5, I updated the
parallel query documentation to reflect recently-committed parallel
query features.  However, a few more things got committed after that.
Most of the attached patch consists of generalizing references to
Gather to also include Gather Merge, but also included a tweak to note
that uncorrelated subplans are no longer parallel-restricted and made
a few other minor improvements and clarifications.

Barring objections, I'd like to commit this in the next couple of days
so that it is included in beta3.  I probably should have gotten to do
this sooner; apologies for any inconvenience.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company