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_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/923241acb15659c8fbbadeab1a6ce7b687672fd3

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