doc: adjust PG 12 release note sections

Bruce Momjian <bruce@momjian.us>

Commit: a6927996be59ac329d65d41cf058d0b1b0978eeb
Author: Bruce Momjian <bruce@momjian.us>
Date: 2019-05-13T03:41:53Z
Releases: 12.0
doc:  adjust PG 12 release note sections

Tighten section designations.

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doc/src/sgml/release-12.sgml modified +29 −38

Documentation touched