C comment update: relfilenode is only unique with a tablespace

Report from Antonin Houska

src/include/storage/relfilenode.h

@@ -55,7 +55,7 @@ typedef enum ForkNumber
  * relNode identifies the specific relation.  relNode corresponds to
  * pg_class.relfilenode (NOT pg_class.oid, because we need to be able
  * to assign new physical files to relations in some situations).
- * Notice that relNode is only unique within a particular database.
+ * Notice that relNode is only unique within a particular tablespace.
  *
  * Note: spcNode must be GLOBALTABLESPACE_OID if and only if dbNode is
  * zero.  We support shared relations only in the "global" tablespace.