Add forwarder callout (smartcontractkit#1767)

Author: thedriftofwords

src/content/chainlink-automation/guides/migrate-to-v2.mdx

@@ -12,6 +12,13 @@ You can migrate most upkeeps that use Automation version 1.2 and later in the [C
 
 For upkeeps on older registry versions 1.0 (Ethereum Mainnet), and 1.1 (BNB Mainnet and Polygon Mainnet), you must migrate manually by cancelling and re-registering your upkeep in the [Chainlink Automation App](https://automation.chain.link/). After you do this manual migration, future migrations will be easier because your new upkeeps will be eligible for the simpler migration process.
 
+<Aside type="caution" title="Grant permission to the forwarder address">
+After you migrate, your new upkeep has a [unique forwarder address](#unique-forwarder) to increase security. This address will be the unique `msg.sender` for your upkeep.
+
+If your upkeep is restricted to a single address calling it, you must give permission to the forwarder address. Otherwise, Automation will no longer be able to execute your function.
+
+</Aside>
+
 ## Migrating using the Chainlink Automation App
 
 The Chainlink Automation App offers a streamlined migration process for upkeeps using registry versions 1.2 and later. To migrate upkeeps with older versions, follow the [manual migration process](#migrating-older-upkeeps-manually) instead.