add docs

Author: sungchun12

docs/guides/linter.md

@@ -74,6 +74,8 @@ Here are all of SQLMesh's built-in linting rules:
 | `invalidselectstarexpansion` | Correctness | The query's top-level selection may be `SELECT *`, but only if SQLMesh can expand the `SELECT *` into individual columns |
 | `noselectstar`               | Stylistic   | The query's top-level selection may not be `SELECT *`, even if SQLMesh can expand the `SELECT *` into individual columns |
 | `nomissingaudits`             | Governance  | SQLMesh did not find any `audits` in the model's configuration to test data quality.                                                 |
+| `nomissingexternalmodels`   | Governance  | All external models must be registered in the external_models.yaml file                                                |
+| `cronvalidator`             | Governance  | Upstream model has a cron expression with longer intervals than downstream model.                                                 |
 
 ### User-defined rules
 

sqlmesh/core/linter/rules/builtin.py

@@ -275,7 +275,7 @@ def create_fix(self, model_name: str) -> t.Optional[Fix]:
 
 
 class CronValidator(Rule):
-    """Upstream model has a cron expression with longer intervals than this model's."""
+    """Upstream model has a cron expression with longer intervals than downstream model."""
 
     def check_model(self, model: Model) -> t.Optional[RuleViolation]:
         placeholder_start_date = "2020-01-01 10:00:00"