docs: clarify primary key uniqueness requirement. (#503)

Not globally unique across tables.

docs/reference/roadmap.md

@@ -89,11 +89,11 @@ By understanding and acknowledging these constraints, you can leverage ElectricS
 
 ### Uniqueness constraints
 
-#### Primary keys need to be globally unique
+#### Primary keys need to be unique
 
-In centralised databases, the creation of two rows with the same primary key causes one of the transactions to abort. In local-first applications, that conflict would only be detected after-the-fact, leaving the state of clients unreconcilable. In ElectricSQL, we require that each key can only be created by a single client at any point in time, ensuring that primary keys are globally unique.
+In centralised databases, the creation of two rows with the same primary key causes one of the transactions to abort. In local-first applications, that conflict would only be detected after-the-fact, leaving the state of clients unreconcilable. In ElectricSQL, we require that each key can only be created by a single client at any point in time, ensuring that primary keys are unique (to their table -- rows in different tables can have the same primary key value).
 
-To ensure that primary keys are globally unique, you can use:
+To ensure that primary keys are unique, you can use:
 
 - UUIDs are typically a safe approach (be aware of limitation with some browsers and devices [REF])
 - Unique data about the client for generating the value safely. E.g., use the clientId in a composed primary-key.