chore: add mention of snapshot format change in upgrade guide (#13038)

Author: SimenB

docs/UpgradingToJest29.md

@@ -14,3 +14,16 @@ See [changelog](https://github.com/facebook/jest/blob/main/CHANGELOG.md#2900) fo
 ## Compatibility
 
 The supported Node versions are 14.15, 16.10, 18.0 and above.
+
+## Snapshot format
+
+As announced in the [Jest 28 blog post](/blog/2022/04/25/jest-28#future), Jets 29 has changed the default snapshot formatting to `{escapeString: false, printBasicPrototype: false}`.
+
+If you want to keep the old behavior, you can set the `snapshotFormat` property to:
+
+```diff
++ snapshotFormat: {
++   escapeString: true,
++   printBasicPrototype: true
++ }
+```