pgsql: Doc: mention platform-dependent behavior of bitshift operators

Doc: mention platform-dependent behavior of bitshift operators

There have been several recent complaints that << and >> don't behave
the same way on all supported platforms. There is a risk that <<
overflows the signed integer type, which is undefined behavior, and there
are platform-dependent behaviors from shifting by numbers of bits >= the
bitwidth of the type, and also from shifting by negative values.

Document this to warn unsuspecting users of these facts.

Author: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAApHDvr0O6ZF8NO=Rq9Q+Sz=FHT=NZ1kkNTXZDcNph-OWuk7TQ@mail.gmail.com
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d6896c056d69da2a451e032cd7b9823de299cdfb

Modified Files
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doc/src/sgml/func.sgml | 7 +++++++
1 file changed, 7 insertions(+)