doc:  update PG 12 release notes, v2
doc:  update PG 12 release notes, v2

Adjustments requested by reviewers.

Reported-by: Amit Kapila, Thomas Munro, Andrew Gierth, Amit Langote, Oleg Bartunov, Michael Paquier, Alvaro Herrera, Tatsuo Ishii

Discussion: 786810e2e6e4a12334bdbc9fa67ac6f0d3aa8832@momjian.us

50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb