pgsql: Document that an SQL-standard-compliant query that expects a uni

Document that an SQL-standard-compliant query that expects a unique row
from information_schema.referential_constraints might return several in
Postgres because we allow duplicate-named constraints in the same
schema.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e07ae21f7ec2260c17136a7afaa4dbca405d9c24

Modified Files
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doc/src/sgml/information_schema.sgml |   13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)