pgsql: doc: Mention AttributeRelationId in FDW validator function descr

doc: Mention AttributeRelationId in FDW validator function description

The documentation has been missing one value in the list of catalog OIDs
that can be given to the validator function of a FDW, as of
AttributeRelationId, when changing the attribute options of a foreign
table.

Author: Ian Lawrence Barwick
Discussion: https://postgr.es/m/CAB8KJ=i16t2yJU_Pq2Z+hnNGWFhagp_bJmzxHZu3ZkOjZm-+rQ@mail.gmail.com
Backpatch-through: 12

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/020e4a17f0ff53129f6e0b739395a524db231bc4

Modified Files
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doc/src/sgml/fdwhandler.sgml | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)