Document relationship key and uniqueness constraints (#225)

Feature merged in 5.3 but hidden behind feature flag, the flag was turned true by default in 5.4.

Author: Hunterness

modules/ROOT/pages/constraints/syntax.adoc

@@ -310,4 +310,4 @@ This means its default behavior is to throw an error if an attempt is made to dr
 With the `IF EXISTS` flag, no error is thrown and nothing happens should the constraint not exist.
 Instead, an informational notification is returned detailing that the constraint does not exist.
 
-For examples on how to drop constraints, see  xref:constraints/managing-constraints.adoc#drop-constraint[Create, show, and drop constraints -> DROP CONSTRAINT].
+For examples on how to drop constraints, see  xref:constraints/managing-constraints.adoc#drop-constraint[Create, show, and drop constraints -> DROP CONSTRAINT].