doc: add named/unnnamed explanation

fix #57

website/guide/pattern-syntax.md

@@ -134,4 +134,10 @@ Note in the example above, even if two meta variables have the same name `$_FUNC
 
 :::info Why use non-capturing match?
 This is a useful trick to micro-optimize pattern matching speed, since we don't need to create a [HashMap](https://doc.rust-lang.org/stable/std/collections/struct.HashMap.html) for bookkeeping.
-:::
+:::
+
+## Capture unnamed node
+A meta variable pattern `$META` will capture [named nodes](/advanced/core-concepts.html#named-vs-unnamed) by default.
+To capture [unnamed nodes](/advanced/core-concepts.html#named-vs-unnamed), you can use double dollar sign `$$VAR`.
+
+Namedness is an advanced topic in [Tree-sitter](https://tree-sitter.github.io/tree-sitter/using-parsers#named-vs-anonymous-nodes). You can read this [in-depth guide](/advanced/core-concepts.html) for more background.