doc: add explanation for constraints

fix #537

website/guide/project/lint-rule.md

@@ -73,6 +73,10 @@ So `console.log(name)` will match the above rule, but `console.log('Rem')` will
 
 See [playground](https://ast-grep.github.io/playground.html#eyJtb2RlIjoiQ29uZmlnIiwibGFuZyI6ImphdmFzY3JpcHQiLCJxdWVyeSI6ImNvbnNvbGUubG9nKCRNQVRDSCkiLCJjb25maWciOiIjIENvbmZpZ3VyZSBSdWxlIGluIFlBTUxcbnJ1bGU6XG4gIHBhdHRlcm46IGNvbnNvbGUubG9nKCRHUkVFVClcbmNvbnN0cmFpbnRzOlxuICBHUkVFVDpcbiAgICBraW5kOiBpZGVudGlmaWVyIiwic291cmNlIjoiY29uc29sZS5sb2coJ0hlbGxvIFdvcmxkJylcbmNvbnNvbGUubG9nKGdyZWV0aW5nKVxuIn0=) in action.
 
+:::warning
+Note, constraints only applies to the single meta variable like `$ARG`, not multiple meta variable like `$$$ARGS`.
+:::
+
 :::details `constraints` is applied after `rule` and does not work inside `not`
 `constraints` is a filter to further refine the matched nodes and is applied after the `rule` is matched.
 So the `constraints` field cannot be used inside `not`, for example

website/reference/yaml.md

@@ -61,6 +61,9 @@ rule:
 
 Additional meta variables pattern to filter matches. The key is matched meta variable name without `$`. The value is a [rule object](/reference/rule.html).
 
+**Note, constraints only applies to the single meta variable like `$ARG`,** not multiple meta variable like `$$$ARGS`.
+So the key name must only refer to a single meta variable.
+
 Example:
 
 ```yaml