fix(JSHandle.toString): clearer description for primitives (#993)

This patch:
- updates JSHandle.toString to make a nicer description for primitives
- excludes JSHandle.toString from documentation to avoid its abuse

References #382

utils/doclint/check_public_api/index.js

@@ -37,6 +37,7 @@ const EXCLUDE_CLASSES = new Set([
 const EXCLUDE_METHODS = new Set([
   'Headers.fromPayload',
   'Page.create',
+  'JSHandle.toString',
 ]);
 
 /**