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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dff1756c392ec7e3ac9b0b4bb2112a594765902a

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(-)