doc: update PG 13 release notes after first draft

Bruce Momjian <bruce@momjian.us>

Commit: 41297fa7d86d0410efc90aebdd39f686b81bf09e
Author: Bruce Momjian <bruce@momjian.us>
Date: 2020-05-05T14:32:52Z
Releases: 13.0
doc:  update PG 13 release notes after first draft

Minor corrections from individuals.

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doc/src/sgml/release-13.sgml modified +38 −23

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