Update version 11 release notes.
Update version 11 release notes.

Remove description of commit 1944cdc98, which has now been back-patched
so it's not relevant to v11 any longer.  Add descriptions of other
recent commits that seemed worth mentioning.

I marked the update as stopping at 2018-07-30, because it's unclear
whether d06eebce5 will be allowed to stay in v11, and I didn't feel like
putting effort into writing a description of it yet.  If it does stay,
I think it will deserve mention in the Source Code section.

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