pgsql: Doc: extend warnings about collation-mismatch hazards in postgre

Doc: extend warnings about collation-mismatch hazards in postgres_fdw.

Be a little more vocal about the risks of remote collations not
matching local ones.  Actually fixing these risks seems hard,
and I've given up on the idea that it might be back-patchable.
So the best we can do for the back branches is add documentation.

Per discussion of bug #16583 from Jiří Fejfar.

Discussion: https://postgr.es/m/2438715.1632510693@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/97c5651cea0f98f281131f3a1f970f875f5951a6

Modified Files
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doc/src/sgml/postgres-fdw.sgml | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)