pgsql: doc: update PG 13 release notes after first draft

doc: update PG 13 release notes after first draft

Minor corrections from individuals.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/41297fa7d86d0410efc90aebdd39f686b81bf09e

Modified Files
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doc/src/sgml/release-13.sgml | 61 +++++++++++++++++++++++++++-----------------
1 file changed, 38 insertions(+), 23 deletions(-)