Updates SnapshotTesting.md to provide more info. on snapshot scope (#6735)

* Updates SnapshotTesting.md to provide more info. on snapshot scope

The doc doesn't mention where to look for the bugs in the code. I have added extra info. which will clear things to find the bugs and why does test pass when you doesn't add any props in the component used in different files.

As a beginner I thought, test will fail if I change Link component props in the any file that usage test component (in this case Link component).

* Lints file

* Calls out code

Author: kalpeshsingh

docs/SnapshotTesting.md

@@ -39,7 +39,10 @@ exports[`renders correctly 1`] = `
 `;
 ```
 
-The snapshot artifact should be committed alongside code changes, and reviewed as part of your code review process. Jest uses [pretty-format](https://github.com/facebook/jest/tree/master/packages/pretty-format) to make snapshots human-readable during code review. On subsequent test runs Jest will simply compare the rendered output with the previous snapshot. If they match, the test will pass. If they don't match, either the test runner found a bug in your code that should be fixed, or the implementation has changed and the snapshot needs to be updated.
+The snapshot artifact should be committed alongside code changes, and reviewed as part of your code review process. Jest uses [pretty-format](https://github.com/facebook/jest/tree/master/packages/pretty-format) to make snapshots human-readable during code review. On subsequent test runs Jest will simply compare the rendered output with the previous snapshot. If they match, the test will pass. If they don't match, either the test runner found a bug in your code (in this case, it's `<Link>` component) that should be fixed, or the implementation has changed and the snapshot needs to be updated.
+
+> Note: The snapshot is directly scoped to the data you render – in our example it's `<Link />` component with page prop passed to it. This implies that even if any other file has missing props (Say, `App.js`) in the `<Link />` component, it will still pass the test as the test doesn't know the usage of `<Link />` component and it's scoped only to the `Link.react.js`.  
+> Also, Rendering the same component with different props in other snapshot test will not affect the first one, as the tests don't know about each other.
 
 More information on how snapshot testing works and why we built it can be found on the [release blog post](https://jestjs.io/blog/2016/07/27/jest-14.html). We recommend reading [this blog post](http://benmccormick.org/2016/09/19/testing-with-jest-snapshots-first-impressions/) to get a good sense of when you should use snapshot testing. We also recommend watching this [egghead video](https://egghead.io/lessons/javascript-use-jest-s-snapshot-testing-feature?pl=testing-javascript-with-jest-a36c4074) on Snapshot Testing with Jest.