pgsql: 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.

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1623740d841fde74dd492fef87d48f0100bdcce8

Modified Files
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doc/src/sgml/release-10.sgml |  9 +++------
doc/src/sgml/release-11.sgml | 31 +++++++++++++++++++++++++++++++
2 files changed, 34 insertions(+), 6 deletions(-)