Document "relrewrite" at the top of heap_create_with_catalog()

This parameter has been introduced in 325f2ec555, and it was not
documented contrary to all the other arguments of
heap_create_with_catalog().

Reviewed-by: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Steven Niu <niushiji@gmail.com>
Discussion: https://postgr.es/m/aE--bmEv-gJUTH5v@paquier.xyz

src/backend/catalog/heap.c

@@ -1100,6 +1100,7 @@ AddNewRelationType(const char *typeName,
  *		if false, relacl is always set NULL
  *	allow_system_table_mods: true to allow creation in system namespaces
  *	is_internal: is this a system-generated catalog?
+ *	relrewrite: link to original relation during a table rewrite
  *
  * Output parameters:
  *	typaddress: if not null, gets the object address of the new pg_type entry