docs: clarify default catalog protocol behavior (#11381)

catalogs/protocol-parser/README.md

@@ -1,3 +1,10 @@
 # @pnpm/catalogs.protocol-parser
 
 > Parse catalog protocol specifiers and return the catalog name.
+
+Catalog protocol specifiers start with `catalog:`. The parser reads the value after that prefix as the catalog name, and returns `default` when no name is provided.
+
+## Examples
+
+- `catalog:foo` -> `"foo"`
+- `catalog:` -> `"default"`