pgsql: Doc: mention foreign keys can reference unique indexes

Doc: mention foreign keys can reference unique indexes

We seem to have only documented a foreign key can reference the columns of
a primary key or unique constraint.  Here we adjust the documentation
to mention columns in a non-partial unique index can be mentioned too.

The header comment for transformFkeyCheckAttrs() also didn't mention
unique indexes, so fix that too.  In passing make that header comment
reflect reality in the various other aspects where it deviated from it.

Bug: 18295
Reported-by: Gilles PARC
Author: Laurenz Albe, David Rowley
Discussion: https://www.postgresql.org/message-id/18295-0ed0fac5c9f7b17b%40postgresql.org
Backpatch-through: 12

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/eda1d0dfe681e9b87c06157e2b0b6864344944f1

Modified Files
--------------
doc/src/sgml/ddl.sgml              | 20 ++++++++++----------
doc/src/sgml/ref/create_table.sgml | 19 +++++++++++++++----
src/backend/commands/tablecmds.c   | 14 +++++++++-----
3 files changed, 34 insertions(+), 19 deletions(-)