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

src/backend/commands/tablecmds.c

@@ -12117,15 +12117,19 @@ transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
 /*
  * transformFkeyCheckAttrs -
  *
- *	Make sure that the attributes of a referenced table belong to a unique
- *	(or primary key) constraint.  Return the OID of the index supporting
- *	the constraint, as well as the opclasses associated with the index
- *	columns.
+ *	Validate that the 'attnums' columns in the 'pkrel' relation are valid to
+ *	reference as part of a foreign key constraint.
+ *
+ *	Returns the OID of the unique index supporting the constraint and
+ *	populates the caller-provided 'opclasses' array with the opclasses
+ *	associated with the index columns.
+ *
+ *	Raises an ERROR on validation failure.
  */
 static Oid
 transformFkeyCheckAttrs(Relation pkrel,
 						int numattrs, int16 *attnums,
-						Oid *opclasses) /* output parameter */
+						Oid *opclasses)
 {
 	Oid			indexoid = InvalidOid;
 	bool		found = false;