pgsql: doc: Remove more notes about compatibilities with past versions

doc: Remove more notes about compatibilities with past versions

This is a follow-up of the work done in fa42c2e, that did not include
all the fixes previously agreed on.  The contents removed here can be
confusing to the reader as they refer to rather old server versions.

Author: Stephen Frost, Tom Lane, Heikki Linnakangas, Yaroslav Schekin
Discussion: https://postgr.es/m/CAB8KJ=jYHgnxLLZSNJz7gBTck4TxomngCmGkw3nEMSNF0yL6wA@mail.gmail.com
Discussion: https://postgr.es/m/1599765595731-0.post@n3.nabble.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8a17f44c1e7a9f3d2a9da97dc3eba4184a2a453c

Modified Files
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doc/src/sgml/func.sgml       | 19 ++++++-------------
doc/src/sgml/gin.sgml        | 10 +++++-----
doc/src/sgml/ref/select.sgml | 12 ------------
3 files changed, 11 insertions(+), 30 deletions(-)