Document some new parallel query capabilities.

This updates the text for parallel index scan, parallel index-only
scan, parallel bitmap heap scan, and parallel merge join.  It also
expands the discussion of parallel joins slightly.

Discussion: http://postgr.es/m/CA+TgmoZnCUoM31w3w7JSakVQJQOtcuTyX=HLUr-X1rto2=2bjw@mail.gmail.com