Last-minute updates for release notes.

Last-minute updates for release notes.

I removed the item about the pg_stat_statements change from
release-11.sgml, as part of a sweep to delete items already committed
in 11.0; but actually we'd best keep it to ensure that people who've
pg_upgraded their databases will take the requisite action.  Also make
said action more visible by making it into its own para.  Noted by
Jonathan Katz.