pgsql: doc PG relnotes: add paragraph explaining the section symbol

doc PG relnotes:  add paragraph explaining the section symbol

And suppress the symbol in print mode, where the section symbol does not
appear.

Discussion: https://postgr.es/m/ZuobILbmGGetxEg5@momjian.us

Backpatch-through: 12

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/00c8c4180a385ced340d52fed41b34ee83731ce4

Modified Files
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doc/src/sgml/release.sgml      | 9 +++++++++
doc/src/sgml/stylesheet-fo.xsl | 5 +++++
2 files changed, 14 insertions(+)