Set comment for fk field if db_constraint is False (#1679)

tortoise/backends/base/schema_generator.py

@@ -265,7 +265,7 @@ def _get_table_sql(self, model: "Type[Model]", safe: bool = True) -> dict:
                     nullable=nullable,
                     unique=unique,
                     is_primary_key=field_object.pk,
-                    comment="",
+                    comment=comment if not reference.db_constraint else "",
                     default=default,
                 ) + (
                     self._create_fk_string(